Annihilation Rate Constant Research Articles

Excited singlet yield in T-T annihilation, a comparative study of naphthalene and anthracene in solution

A method based on the study of both triplet absorption and delayed fluorescence (DF) decays after flash excitation has been developed to determine DF yields. Values of the excited singlet yield in the T-T annihilation process have been obtained for naphthalene and anthracene in solution at room temperature by application of this method. Triplet extinction coefficients and annihilation rate constants for these compounds were also determined. Excited singlet generation is the predominant annihilation channel for naphthalene whereas it is only of minor importance for anthracene. The difference in DF yields is ascribed to differences between these aromatics with respect to the efficiencies of the annihilation channels leading respectively to excited singlet and triplet states. The higher rate of excited singlet formation for naphthalene is qualitatively accounted for by considering the possibility of radiationless transitions from the bimolecular states formed in T-T interaction towards an upper excimer singlet state.

Carrier-Exciton Interactions in Crystalline Anthracene

Quenching of delayed fluorescence by injected holes and electrons is observed when either monomolecular or bimolecular exciton processes afford the dominant exciton decay mechanism. With small triplet exciton densities, injected carriers reduce the triplet exciton lifetime, and hence the delayed fluorescence intensity. With high triplet exciton densities this effect is unimportant; however, the over-all triplet-triplet annihilation rate constant is found to increase while the singlet component of the annihilation rate constant decreases in the presence of charge carriers. These effects are discussed in terms of triplet pair formation and annihilation.

Triplet State of Acetylene: Biacetyl Emission from the Mercury Photosensitized Reaction

The question has been studied whether a metastable triplet state of acetylene is formed during its mercury photosensitized reaction, using biacetyl as a probe and employing a modulation-phase shift technique. While no emission is observable in the mercury photosensitized reactions of acetylene or biacetyl alone, in the presence of both compounds biacetyl phosphorescence is generated whose intensity, under suitable conditions of pressure and flow, approaches closely the one of benzene photosensitized biacetyl emission. The phenomenon is not caused by products such as benzene and can be quenched by increasing the acetylene pressure. A mechanism explaining the emission as energy transfer from a metastable acetylene triplet state to biacetyl is proposed, and rate parameters are evaluated by comparing this model with the experimental results. Self-quenching of triplet acetylene occurs at 0.083 times the rate of energy transfer to biacetyl. The excitation energy of triplet acetylene, E, was estimated to be 2.6<E<4.7 eV on the basis of the biacetyl phosphorescence yield and quenching experiments with other triplet states. In addition, the results provide an estimate, viz., 2× 1011 M−1·sec−1, for the biacetyl triplet-triplet annihilation rate constant.

Effects of exciton interactions on the fluorescence yield of crystalline tetracene

The relative fluorescence yield of crystalline tetracene as a function of the intensity of weakly absorbed light has been measured. The yield increases, due to the contribution of delayed fluorescence, with increasing intensities by a factor of 2.90 from the prompt fluorescence value. The value inferred for the total triplet-triplet annihilation rate constant is γ TT = (2.2±0.5)×10 −9 cm 3 sec −1, and for the ratio of the rate constant of triplet fusions generating singlets to γ TT is f = 0.66±0.06. This last value shows that the triplet channel is significantly less important than predicted by simple spin statistics. At higher intensities the yield decreases with increasing intensities. The effect is attributed to quenching of singlets by singlet-triplet annihilation yielding an estimate of γ ST ≈ 2 × 10 −7 cm 3 sec −1 for the rate constant of the interaction.

Annihilation of singlet excited states in anthracene solutions

The quenching of anthracene fluorescence in benzene solutions under high UV light intensity has been studied using a pulsed nitrogen laser (γ = 3371Å). It is shown to result from singlet-singlet and singlet-triplet annihilation. The overall annihilation rate constant is (3.5 ± 1.5) × 10 −10 cm −3 sec −1. Annihilation is due to dipole-dipole interaction of excited molecules.

Comparison of high and zero magnetic field values fo non radiative triplet-triplet annihilation rate constants in crystalline anthracene

Experimental indications are given which show that the non-radiative triplet-triplet annihilation rate constant (ARC) γ nr in anthracene has the same value at high (3 kOe) and zero magnetic field. From the anisotropy of the negative magnetic field effect on the electron-hole recombination emitted light, it is also shown that γ nr/γ r = 1.5 ± 0.3, where γ r is the radiative ARC. Electron-hole recombination, triplet and singlet creation yields in the recombination process are also identical at high and zero fields.

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.

Laser Generation of Excitons and Fluorescence in Anthracene Crystals

Experimental and theoretical studies are reported of the short-lived and delayed fluorescence of anthracene single crystals, excited by single- and double-photon absorption. A giant-pulse ruby laser provides the primary source of radiation of 14 400 cm−1 (up to 1027 photons/cm2·sec) and is also used to generate second-harmonic radiation from ADP, as well as stimulated Raman radiation of 12 800 and 17 500 cm−1 from liquid oxygen. The time dependence of the fluorescence intensity is studied as a function of laser intensity, crystal temperature and excitation wavelength. The very intense fast fluorescence with a half-life of 30 nsec at 300°K, characteristic of singlet exciton decay, and the relatively weak delayed fluorescence which involves intermediate triplet states, are separated using sectored disks. It is concluded that the triplet state at 14 750 cm−1 can be populated (i) by direct absorption of laser photons involving an activation energy of 350 cm−1; (ii) via two-photon absorption, presumably leading to a vibrationally excited state of the 1B2u exciton, followed by intersystem crossing; (iii) via one-photon (second-harmonic) excitation from levels≥700 cm−1 into the singlet absorption band, followed by conversion of the singlet exciton into a triplet pair. The latter process is suggested by the observed activation energy of 700 cm−1. In agreement with these interpretations, the delayed fluorescence intensity is found to vary with the second to fourth power of the laser intensity depending on the experimental conditions. Also, light of 17 500 cm−1 leads exclusively to Process (i), light of 12 800 cm−1 exclusively to (ii). Triplet lifetimes from 2–17 msec are obtained, depending on crystal purity, which indicates that unimolecular triplet decay is an extrinsic, radiationless process. A singlet—triplet intersystem crossing rate constant of about 3×10−5 sec−1 is estimated. The triplet—triplet annihilation rate constant is found to be about 5×10−11 cm3 sec−1. This value considered together with the triplet-pair creation process suggests a triplet exchange rate ≳ 1013 sec−1 and a triplet diffusion constant ≳o5×10−4cm2/sec.