Radiative Decay Rate Constants Research Articles

Electronic relaxation in isolated substituted benzenes

Data on the rate constants for radiative and non-radiative decay of isolated substituted benzenes as a function of excess energy are considered with reference to current theories. The possible role of vibrational redistribution in these isolated molecules is discussed, and the assumptions inherent in the use of theoretical models stressed.

Die kinetischen Konstanten des Triplett-Zustandes von Fluoren-Molekülen / The Cinetic Constants of the Triplet Stale of Fluorene Molecules

Abstract The metastable triplet state T1 of fluorene molecules in a fluorene crystal, disturbed by the presence of dibenzothiophene (“X-traps”) was investigated at 1.6 and 4.2 °K. From ESR measurements a model for the fluoren X-traps is derived. The X-trap molecule is misoriented by an angle of 2,5° in the fluorene crystal. The rate constants for population (s), radiative decay (kD) and total decay (k) of the three magnetic sublevels of T1 were determined by analysis of phosphorescence intensity and decay time in high magnetic field and by optically detected ESR. We find (with z = axis perpendicular to the molecule, x = long axis): The average spin lattice relaxation rate constant is w = (0.8 ± 0.2) sec- 1 .

Photophysics of substituted benzenes. Absolute rate constants for singlet radiative and non-radiative decay in the vapour phase

Fluorescence decay times for the vapour of several fluoro-, methyl- and trifluoromethyl-benzenes have been determined at various exciting wavelengths and pressures, and absolute rate constants for radiative and non-radiative decay of the first excited singlet states of these compounds calculated. For fluorine substitution, values of the radiative rate constant are greater than those obtained by integration of absorption curves. With increase in photon energy, the non-radiative rate constant increases and, generally, the radiative rate constant decreases in magnitude.

Photochemistry of trifluoromethyl benzenes IV. 1-fluoro-2-(trifluoromethyl)- and 1-fluoro-3-(trifluoromethyl)-benzenes

Quantum yields of fluorescence and intersystem crossing for the vapour of 1-fluoro-2-(trifluoromethyl)- and 1-fluoro-3-(trifluoromethyl)-benzene excited at various wavelengths corresponding to absorption in the first singlet band have been measured. Absolute rate constants for radiative and non-radiative decay of these compounds based upon these measurements and fluorescence decay times were derived, and data were compared with other substituted benzenes. Rate parameters for vibrational relaxation and quenching of the excited singlet and triplet states of the aromatic molecules were also measured.

Zeeman effect in phosphorescent molecules

The T → S o phcsphorescence of isolated and identically oriented quinoxaline molecules was studied in high magnetic fields. The intensity ratios of the three Zeeman components are a function of the field direction. They are used to determine the relative values for the radiative decay rate constants of the three zero .field levels of the triplet state: k z D/ k x D > 50, k z D/ k y D > 50.

Temperature Effects on the Phosphorescence of Aromatic Hydrocarbons in Poly(methylmethacrylate)

Temperature effects on the phosphorescence lifetimes and phosphorescence intensities of naphthalene, phenanthrene, pyrene, biphenyl, and the perdeuterated analogs of these four molecules were measured with the use of poly(methylmethacrylate) (PMM) as a rigid solvent matrix. The phosphorescence lifetimes of these molecules decreased by only 13%–37% upon warming from 77°K to room temperature. On the basis of direct measurements of the permeability of PMM to oxygen, we conclude that oxygen quenching is not responsible for the observed temperature effects on the phosphorescence lifetimes. Although we cannot exclude the possibility that the rate constants for both radiative and nonradiative decay vary with temperature, we show that our results are consistent with an analysis that assumes that only the non-radiative rate constant is temperature dependent. On the basis of this analysis, we estimate an activation energy of 500 ± 100 cm−1 for the thermally activated nonradiative decay mode.