Energy Trapping Processes in Aromatic Crystals

Excitons in ideal aromatic crystals are delocalized. So far no experimental evidence for self trapping of excitons is known. Trapping of energy is possible in mixed crystals or in crystals containing defects. Disturbed exciton states due to the presence of foreign molecules or due to structural defects (the so-called X-traps) are the most characteristic type of defects in aromatic crystals. Some spectroscopic experimental data on X-traps are collected and discussed. The most sensitive method for detecting traps is that of sensitized delayed fluorescence. The kinetics of sensitized delayed fluorescence is• discussed and compared with that of prompt fluorescence. Trap concentrations as low as l0b0 mol/mol can be detected using delayed fluorescence. I. THE IDEAL CRYSTAL The energy levels of an ideal aromatic crystal, like naphthalene or anthracene, lie in bands: valence bands, exciton bands and conduction bands. Energy is completely delocalized in the ideal crystal. The time scale for transitions is determined by the electronic structure of these bands, and by the transition matrix elements. Transitions between these crystal states are: Absorption from the ground state S0 to excited states S,. Fluorescence from the lowest excited singlet state S1 to S0. The lifetime of S1 is typically between 5 and 500 nsec. Internal conversion between different excited S states or triplet (7) states. These processes occur in less than 0.1 nsec. Intersystem crossing between the singlet and the triplet manifold. The typical time constants are 100—1000 nsec. Phosphorescence from the lowest excited triplet state T1 to the ground state S0. The lifetime of T1 is typically between 1 and iO msec and is determined by radiationless processes. Energy storage for times longer than 0.5 gtsec is possible only in the T1 band. Energy relaxation between the energy bands (internal conversion) is due to exciton—phonon interaction. Trapping processes, which are able to localize energy at specific sites in the perfect crystal are unknown so far. There is no experimental evidence for the self trapped exciton, which was first discussed by FrenkeP, and later by Sidman2.