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M. Gehring, B. Nickel

Delayed Excimer Fluorescence of Fluoranthene Due to Triplet-Triplet Annihilation: Systematic Study of the Fluorescence from a Weakly Bound Excimer

Solutions of fluoranthene exhibit a delayed fluorescence (DF) due to triplet-triplet annihilation, ³M* + ³M* → ¹M* + ¹M (P-type DF). In the fluid solvent 3-methylpentane, at low temperatures, the DF consists of two components: delayed monomer fluorescence (DMF) and a slightly red-shifted component, which is assigned to delayed excimer fluorescence (DEF). The temperature dependence of the intensity ratio Q_EM of DEF and DMF was measured between 133 K and 293 K. The assignment of the second component of the DF to the DEF of fluoranthene is consistent with the rise and the decay of the DF at 153 K. A kinetic model was developed that describes the reversible geminate association of a ¹M*···¹M pair to an excimer, ¹M*···¹M⇔¹(MM)*. The main features of the kinetic model are: a Smoluchowski equation, a distance-dependent relative diffusion coefficient, an exponential distance dependence of the first-order rate coefficient for the annihilation of a triplet pair, and a short-range potential representing the excimer. The Smoluchowski equation was numerically solved with a new algorithm, which is based on numerical fundamental solutions and allows the successive doubling of the total diffusion time. The kinetic model accurately describes Q_EM(T) from ≈150 K (maximum of Q_EM(T)) to room temperature. The effective first-order rate constant for excimer dissociation, k_diss(T), the equilibrium constant K_EM(T) for excimer formation, and the excimer binding enthalpy are estimated. The observability of prompt excimer fluorescence is discussed. A formation enthalpy of the ground-state dimer of ≈-hc×500 cm^-1 is estimated. The major part of the DEF spectrum is approximately equal to a spectrum, which is produced by inhomogeneous spectral broadening (i.e. convolution with a Gaussian) and a red-shift of the spectrum of the monomer fluorescence. The DEF spectrum contains no band that corresponds to the 0-0 transition of the monomer fluorescence.

Zeitschrift für Physikalische Chemie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0942-9352
Volume: 215, 03/2001
Pages: 343

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