Photoluminescence of pentacene single crystals is studied in the temperature range of 7 K to 200 K under excitation with He-Ne laser light. Photoluminescence spectra consist of four broad bands, L1 to L4. The highest energy band, L1, located close to the lowest exciton absorption band mainly appears for //b-polarization. The intensity of the second one, L2, with Stokes-shift of about 1500 cm -1, decreases as temperature rises above 30 K and disappears at 100 K. The bands, L3 and L4, which are located at lower energy, are observed at higher temperatures up to 200 K. Based on their energy positions, the band L2 is assigned to a shallow self-trapped exciton luminescence band, and the bands L3 and L4 to deep self-trapped exciton luminescence bands. By comparing this result with reported result on tetracene crystals, self-trapped excitons are considered to be more stable in pentacene.

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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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