Optical properties of single fluorene/phenanthrene crystals doped with various mole fraction of anthracene, 10 -5 to a few %, were investigated at 10 K. In low concentration samples, sharp structures attributed to isolated anthracene molecules were observed. Inhomogeneous width in their 0-0 energy were found to be ~20 cm -1 . When concentration is higher, spectral species, which are distinctly different from the isolated molecule spectrum, were observed and their origins are discussed. . For microcrystallites consisting of small number of molecules, appreciable energy gaps in the lowest electronic excitation level (0-0) are expected between crystallites consisting of n and n+1 molecules. However, gaps are considered to be difficult to observe with PMMA as a matrix, since 0-0 energies of crystallites of the same size would have distribution width due to inevitable local inhomogeneity. In this study, to achieve uniform environment, single crystals of fluorene and phenanthrene were chosen as matrices. Optical properties were investigated for various anthracene mole fraction samples (10 -5 to a few %) to estimate inhomogeneous width in low concentration samples and to search spectral species different from the one from isolated molecules in high concentration samples.
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