Excitation dynamics in single molecular crystals of α-hexathiophene from femtosecond to millisecond

We identify odd- and even-parity states and measure transient photomodulation spectra of Frenkel excitons in a-hexathiophene single crystals. The lowest Frenkel excitons are characterized by a stimulated emission band at ~2 eV and an absorption band at 1.3 eV. Intramolecular internal conversion governs transitions from upper excited states to the lowest exciton and occurs in 200 fs. We observe exciton trapping on the nanosecond time scale; as a result, the millisecond photomodulation spectrum is dominated by the absorption of trap states. Using photoinduced-absorption-detected magnetic resonance spectroscopy, we identify the excited-state absorption spectra of triplet excitons and polarons, both of which are produced with a low quantum yield. The triplet absorption band is structureless and approximately 4 times wider than the absorption of singlet excitons. The long-lived photoexcitations are also studied by the photoluminescence-detected magnetic resonance (PLDMR) technique, as a function of the magnetic field orientation in the crystal b-c plane. We find that there exist two triplets with similar zero-field-splitting parameters, but with the principal axes oriented along different directions, giving rise to six magnetic resonances in the PLDMR spectra.