Luminescence Properties and Energy Transfer Processes in Fluorescent and Phosphorescent Tris(Phenylquinoxaline)
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raction (singlet-singlet transfer) and electron exchange (triplet-triplet transfer) according to Förster’s and Dexter’s theories [7,8], respectively . In order to observe efficient energ y transfer it is necessary that the emission spectrum of the h o s t overlaps with the absorption spectrum of the guest and that the energy levels of the singlet and triplet state of the h o s t lie above the respective levels of the guest. The h o s t materials used so far in electrophosphorescent devices are not phosphorescent, thus the energetic position of the triplet state is not known and predictions about the efficiency of energy transfer of triplet excitons are hard to make. Besides this, Dexter transfer is a ’physical’ transfer - it requires an overlap of the wavefunctions of host and guest molecules and thus high doping concentrations in order to be efficient which may lead on the other side to undesired concentration quenching effects. It seemed to us therefore interesting to chose as active layer for an OLED a h o s t material which both fluoresces and phosphoresces , first, to be able to evaluate energy transfer processes from the excited singlet as well as triplet state of the (charge carrier transporting) host to an emitting guest molecule , and second, since in the case of a phosphorescent host, tripletsinglet energy transfer via long range dipole -dipole interaction is possible . Recently , a new class of electron transport materials – the tris(phenylquinoxa line)s (TPQ)- has been reported, and it has been shown that introduction of a thin layer of those molecules in an OLED enhances the EL efficiency considerably due to its favourable electron transporting and hole blocking ability [9]. Besides this, TPQs have the desired PL properties - they emit both fluorescence and phosphorescence and therefore allow us to study energy transfer processes involving both singlet and triplet excitons. As a guest molecule we have chosen a red emitting phosphorescent dye -2,3,7,8,12,13,17,18octaethyl-21H,23H- porphin platinum (II) (PtOEP). This molecule has a very high phosphorescence quantum yield , (ca. 50 % in a polystyrene matrixat 300 K) [10] and the energy levels of its excited states match the respective levels of those of TPQ sufficiently. PtOEP, as mentioned above, has been used already as emitter and showed very good performance in electrophosphorescent OLEDs. In order to study energy transfer processes and estimate their efficiency we have measured absorption, photoluminescence excitation, steady state and time resolved PL spectra of pristine and doped thin TPQ films and recorded EL spectra . EXPERIMENTAL The structure of PtOEP and 1,3,5-tris [(3-phenyl-6-trifluoremethyl) quinoxaline2-yl]benzene, the TPQ used here is shown in Figure 1. W e have chosen a TPQ with a fluoromethyl substituent, since its phosphorescence quantum yield is higher than for those with methyl substituents . The synthesis of TPQ has been reported elsewhere [9]. To record photoluminescence (PL) and absorption spectra thin films of TPQ and PtOEP dispers
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