Fluorescence decay time measurements in tetracene crystals excited with synchrotron radiation

Fluorescence decay times from tetracene single crystals excited at room temperature with synchrotron radiation have been recorded as a function of the excitation wavelength (in the 400–500 nm range). A non-exponential decay with two decay rates is observed. The analysis of our data shows that the first singlet exciton level of tetracene (single crystal) decays radiatively mainly through, as we call it, channel 1, with a lifetime of 0.200 ± 0.020 ns. About 10% of the emitted fluorescence transits through channel 2 with a lifetime of 1.7 ± 0.2 ns. These results do not agree with previously published decay data obtained when tetracene is excited by means of powerful lasers. Thus there is experimental evidence to believe that the decay properties of condensed materials can be very dependent on the excitation density. Because synchrotron radiation compared to lasers is a very weak source, and therefore secondary effects are minimized in our experimental conditions, the decay values reported in the present work are the true lifetimes of the tetracene single crystal.