Charge‐Transfer Excitons and Delayed Fluorescence of Anthracene

The paper deals with decay kinetics and the steady state intensity LSO of the delayed fluorescence of anthracene in a magnetic field (H = 3000 G); LSO being proportional to the incident light intensity I. The triplet exciton annihilation rate constant γ, which is proportional to the square of the slope of the kinetic curve γ = dL /dt, is found to decrease as the incident light intensity I increases. Under these conditions a change is observed in the fluorescence decay kinetics from the usual hyperbolic law, characteristic of triplet bimolecular annihilation, to an exponential one with a time constant t′ = 1.6 ms. A conclusion is made about the formation of an intermediate excited state (charge‐transfer (CT) excitons) after triplet‐triplet annihilation but before the delayed singlet exciton M generation. This conclusion is confirmed by the influence of a magnetic field on the delayed fluorescence steady state intensity LSO (ΔSO/LSO = −10%) and on the slope of the decay curve γeff0.5. An investigation of the CT exciton lifetime, which is determined from the delayed fluorescence exponential decay, as a function of crystal temperature and the incident light intensity seems to show that the CT exciton level in anthracene is situated between the wide and narrow conduction bands.