Excitation dynamics in single molecular crystals of α-hexathiophene from femtoseconds to milliseconds

Abstract

We identify odd-and even-parity states and measure transient photomodulation spectra of Frenkel excitons in ␣-hexathiophene single crystals. The lowest Frenkel excitons are characterized by a stimulated emission band at ϳ2 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 ͑PLDMR͒ 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.

Nonradiative relaxation processes in molecular crystals

“…This is in sharp contrast to experimental studies in which such relaxation processes are observed within a time interval of less than 0:5 ps after excitation [3,8]. This is not an artifact of the choice of initial condition, since we always observe the internal conversion process to take place over several hundreds of femtoseconds.…”

“…It is therefore difficult to estimate the exact duration of the relaxation of the electronic system. From, e.g., transient photomodulation spectroscopic studies of photoexcitation dynamics in both ultra pure tetracene crystals [21] and ahexathiophene crystals [3] intramolecular internal conversion is found to take less than 200 fs. As a reference, we note that it takes roughly 1:1 ps for the system to go from the state of occupation when n 116 first drops below 0.50 at t$0:5 ps to that when n 111 rises above 0.50 at t$1:6 ps.…”

“…The prime in Eq. (3) indicates that the summation runs over nearest neighbors only. For the molecular dynamics we adopt a classical description of the s-bond energy.…”

“…Although practical devices are likely to be produced from disordered polycrystalline films, high purity crystals are required for studies of the intrinsic dynamics, since, otherwise, trapping of excitons to impurities can dominate the dynamical process. In particular, a-hexathiophene [3,4], rubrene [5], and pentacene [6,7] have been extensively studied and are considered to be representative model systems for p-conjugated molecular crystals. From such studies the dominant relaxation channel from higher lying excited states was identified to be internal conversion [3], i.e., the ultrafast nonradiative relaxation between molecular states of the same spin multiplicity [8].…”

“…In particular, a-hexathiophene [3,4], rubrene [5], and pentacene [6,7] have been extensively studied and are considered to be representative model systems for p-conjugated molecular crystals. From such studies the dominant relaxation channel from higher lying excited states was identified to be internal conversion [3], i.e., the ultrafast nonradiative relaxation between molecular states of the same spin multiplicity [8]. This process is followed by a much slower decay of the (intra)molecular vibrations on the order of a few picoseconds [4].…”