Excitation dynamics in single molecular crystals of alpha-hexathiophene from femtoseconds to milliseconds - art. no. 205203

We identify odd- and even-parity states and measure transient photomodulati on spectra of Frenkel excitons in alpha -hexathiophene single crystals. The lowest Frenkel excitons an characterized by a stimulated emission band at similar to2 eV and an absorption band at 1.3 eV. Intramolecular internal co nversion governs transitions from upper excited states to the lowest excito n and occurs in 200 fs. We observe exciton trapping on the nanosecond time scale: as a result, the millisecond photomodulation spectrum is dominated b y the absorption of trap states. Using photoinduced-absorption-detected mag netic resonance spectroscopy, we identify the excited-state absorption spec tra of tr triplet excitons and polarons, both of which are produced with a low quantum yield. The triplet absorption band is structureless and approxi mately 4 times wider than the absorption of singlet excitons. The long-live d photoexcitations are also studied by the photoluminescence-detected magne tic resonance (PLDMR) technique, as a function of the magnetic field orient ation in the crystal b-c plane. We find that there exist two triplets with similar zero-field-splitting parameters, but with the principal axes orient ed along different directions, giving rise to six magnetic resonances in th e PLDMR spectra.