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 triple t-triplet annihilation, M-3* + M-3* --> M-1* +M-1 (P-type DF). In the fluid solvent 3-methylpentane, at low temperatures, the DF consists of two compo nents: delayed monomer fluorescence (DMF) and a slightly red-shifted compon ent. which is assigned to delayed excimer fluorescence (DEF). The temperatu re dependence of the intensity ratio Q(EM) of DEF and DMF was measured betw een 133 K and 293 K. The assignment of the second component of the DF to th e DEF of fluoranthene is consistent with the rise and the decay of the DF a t 153 K. A kinetic model was developed that describes the reversible gemina te association of a M-1*...M-1 pair to an excimer, M-1*...M-1=(1)(MM)*. The main features of the kinetic model are: a Smoluchowski equation, a distanc e-dependent relative diffusion coefficient, an exponential distance depende nce of the first-order rate coefficient for the annihilation of a triplet p air. and a short-range potential representing the excimer. The Smoluchowski equation was numerically solved with a new algorithm, which is based on nu merical fundamental solutions and allows the successive doubling of the tot al diffusion time. The kinetic model accurately describes Q(EM)(T) from app roximate to 150 K (maximum of Q(EM)(T)) to room temperature. The effective first-order rate constant for excimer dissociation, k(diss)(T), the equilib rium constant K-EM(T) for excimer formation, and the excimer binding enthal py are estimated. The observability of prompt excimer fluorescence is discu ssed. A formation enthalpy of the ground-state dimer of approximate to -hc x 500 cm(-1) is estimated. The major part of the DEF spectrum is approximat ely equal to a spectrum, which is produced by inhomogeneous spectral broade ning (i.e. convolution with a Gaussian) and a red-shift of the spectrum of the monomer fluorescence. The DEF spectrum contains no band that correspond s to the 0-0 transition of the monomer fluorescence.