Optical properties of anthracene-fluorene mixed crystals fabricated by cell method

Anthracene-fluorene mixed crystals with anthracene mole fraction of 0.1 to 100% are fabricated using a cell method and their optical properties are studied. In contrast to sharp luminescence due to isolated anthracene molecules in a 0.1% doped sample and in a pure anthracene bulk crystal, broad luminescence bands are observed in the middle concentration samples. For samples with less than 50% anthracene, broad luminescence bands are attributable to localized excited state at anthracene molecule pair which is accompanied by lattice relaxation. For samples with higher anthracene concentration, observed broad luminescence has lower peak energy than that of an anthracene bulk crystal. Origin of the luminescence is proposed to be excitation localized at fluorene impurity induced X-traps in anthracene clusters. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)