Triplet Excitons in Organic Materials

The theory of delayed fluorescence produced in an organic material by singlet excitation through an absorbing layer is given. It is shown, that it will be reduced by a factor la.2/ ( la .+ ls) 2 due to the singlet energy transfer to such a layer, where /a the penetration depth of the exciting irradiation and Zs the diffusion lenght of the singlets. Further reduction of delayed fluorescence should be observed, when also triplet excitons transfer energy to the absorbing layer. Assuming this triplet transfer probalitity to be of the same order of magnitude as that for triplet energy transfer between neighbouring molecules, it should lead to quenching of delayed fluorescence proportional to U 2/l t2The exponential portion of the decay of delayed fluorescence should become faster only if triplet energy transfer to the layer takes place. Further, the triplet induced hole-current produced at an absorbing electrode under singlet excitation should display a dependence on the wavelength of the exciting radiation as Za/(^a+Zs)Its maximum should not occur at the wavelength of strongest absorption but at much larger penetration depths of the exciting radiation.