Luminescences of Pyrene Single Crystal and Pyrene Molecules Inserted in a Molecular Vessel of Cyclodextrin

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Highly purified single crystals of pyrene were made by a gas phase crystal growth method from 180 times of zone-refined pyrene. The dry powder of β-cyclodextrin including a pyrene single molecule and that of γ-cyclodextrin including twin molecules were prepared under vacuum. The luminescence spectra of pyrene single molecules, twin molecules and single crystals, and their temperature dependences were measured with great care in the process of cooling the samples. It is found that the features of luminescence spectra at 2 K are quite different from each other. Pyrene twin molecules in γ-cyclodextrin show an excimer luminescence similar to that of single crystal. The vibronic luminescence in the range of 3.0–3.5 eV comes from the ground-excited-state complex, or in other words, from the vibronic exciton, in contrast to the hitherto-assigned luminescence from another type of selftrapped exciton or metastable B-state which is the precursor of selftrapped excitons.