Abstract
Fluorescence spectra of crystalline tetracene have been recorded in the temperature range 120 to 300 K under hydrostatic pressure up to 600 MPa. From discontinuities in both emission spectra and spectral intensities it is concluded that two phase transitions occur. The room temperature phase is transformed to a low temperature phase/high pressure phase I at T I t ( p = 0) = 182 K, the temperature coefficient being d T I t/d p = 0.395 K/MPa. The phase transition is induced by a decrease of the specific volume under pressure and/or upon cooling. Lack of a significant shift of the origin of the fluorescence band near T I t at constant pressure is an artifact resulting from the neglect of reabsorption effects. The Stokes shift is 260 cm -1, independent of temperature and crystal modification. In accord with previous Raman data a second phase transition occurs at T II t ( p) = 143 K, the pressure shift being d T II t/d p = 0.088 K/MPa. In addition, the shift of the triplet energy as a function of pressure as well as the pressure-dependence of the rate constants governing fission of a singlet exciton into a pair of triplets is discussed utilizing their magnetic field dependences.
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