Routes to singlet exciton fission in rubrene crystals and amorphous films

By using wavelength-tunable, low fluence picosecond pulses at a 1 kHz repetition rate in a transient grating pump and probe configuration, we show that photoexcitation to higher vibrational levels leads to slower singlet exciton fission rates in rubrene. After the 1 ps photoexcitation, the initial growth of the triplet exciton population is exponential, with its time-constant systematically varying from 2.5 +/- 0.7 ps to 40 +/- 15 ps as the excitation photon energy is increased from the lowest to the third main vibrational band in the vibrational progression of rubrene's absorption spectrum. We also determine that short-pulse-induced fluorescence in amorphous rubrene films deposited by molecular beam epitaxy in high vacuum decays as a single exponential with a lifetime of 15.2 ns, close to rubrene's radiative lifetime for molecules in solution. This shows that singlet fission is non-existent in fully amorphous rubrene films, despite the close molecular packing, which indicates that in these films there is no short range molecular order matching the molecular arrangement in orthorhombic rubrene crystals.