Hydrocarbons, aromatic delayed fluorescence

P-type delayed fluorescence is so called because it was first observed in pyrene. The fluorescence emission from a number of aromatic hydrocarbons shows two components with identical emission spectra. One component decays at the rate of normal fluorescence and the other has a lifetime approximately half that of phosphorescence. The implication of triplet species in the mechanism is given by the fact that the delayed emission can be induced by triplet sensitisers. The accepted mechanism is ... 

The existence of this TTA bimolecular process makes it possible to carry out cooperative excitation of energy rich molecular states by low energy quanta. In particular, formation of upper excited singlet states as a result of TTA has been observed for several aromatic hydrocarbons (1) and metalloporphyrins ( ) by the demonstration of annihiTation-induced delayed fluorescence (AOF) from upper singlet states. 

A number of dissolved aromatic hydrocarbons exhibit both molecular and excimer bands in the delayed fluorescence spectrum6,83,116,117 that originates118 in the mutual annihilation of two molecular triplet states if this can produce both the molecular singlet state and the excimer directly according to the scheme... 

B) P-type delayed fluorescence is so called because it was first observed in pyrene and phenanthrene solutions. In aromatic hydrocarbons singlet-triplet splitting is large and therefore thermal activation to excited singlet state at room temperature is not possible. The mechanism was first formulated by Parker and Hatchard based on the observation that the intensity of emission of the delayed fluorescence Ipd was proportional to the square of the intensity of absorption of the exciting light Ia. 

This description of the relative spectral linewidths of the lowest excited toi states applies to the whole family of aromatic hydrocarbons. It also applies to the manifold of triplet jui states. In the case of benzene, Burland, Castro and Robinson 24> and Burland and Castro 25> have used phosphorescence and delayed fluorescence excitation techniques, respectively, to measure the absorption spectrum of the lowest triplet state, 3Biu of ultrapure crystals at 4 K. The origin is located at 29647 cm-1. Unlike all the earlier studies on the lowest singlet triplet absorption spectrum, this was not an 02 perturbation experiment. Here widths of less than 3 cm-1 were obtained. This result should be compared with the much broader bands 150-1 observed for the suspected second triplet ZE i in 5 cm crystals of highly purified benzene 26>. The two triplet states are separated by 7300 cm"1. 

Observations of the room temperature phosphorescence of polycyclic aromatic hydrocarbons in micelles, which stabilize the triplet states by reduction of quenching, indicate that measurement of phosphorescence lifetimes can be a useful analytical parameter . Room temperature phosphorescence and delayed fluorescence have been shown to occur with triplet states of a wide variety of organic molecules in silica sol-gel glasses . ... 

Nickel, B., Delayed Fluorescence from Upper Excited Singlet States S (n > 1) of the Aromatic Hydrocarbons 1,2 Benzanthracene, Fluoranthene, Pyrene, and Chrysene in Methylcyclohexane, Helv. Chim. Acta 1978, 61, 198 222. 

Iangelaar, J., Retschnik, R.P.H., and Hoijtink, G. (1971) Studies on triplet radiative lifetimes, phosphorescence, and delayed fluorescence yields of aromatic hydrocarbons in liquid solutions. 

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