Abstract
Microcrystalline pyrene films deposited on silicon substrates are characterized by steady-state and picosecond spectroscopy in the temperature range between 5 and 300 K. The films reveal at high temperatures all the typical features of the single crystal high temperature phase. Upon cooling to low temperatures we find that the majority of the microcrystals does not show a phase transition thus making the crystalline high temperature phase accessible in the temperature range between helium and room temperature. For that whole range the measurements of fluorescence quantum yields and transients quantitatively confirm our concept of two-step excimer formation via the metastable precursor B-state. The same thermal activation barrier between B- and excimer state, (350 ± 30) cm -1, is found as in the single crystal. The intrinsic lifetime of the B-state is measured to be about (200 ± 10) ns in accordance with our previous estimates. From the detailed analysis of the B-fluorescence spectrum at low temperatures we conclude that already the B-state is subject to appreciable electron-phonon coupling.
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