Magnetic Field Effects on Singlet Fission and Fluorescence Decay Dynamics in Amorphous Rubrene

Abstract

Picosecond time-resolved fluorescence experiments are used to study the dynamics of singlet fission in highly disordered films of rubrene. The fluorescence spectral lineshapes are not temperature-dependent, indicating that intermolecular excitonic effects are absent in these films. The temperature-dependent fluorescence decays in the amorphous films are nonexponential, containing both prompt and delayed components. The kinetics are qualitatively consistent with the presence of singlet fission, but to confirm its presence, we examine the effects of magnetic fields on the fluorescence decay. A quantum-kinetic model is developed to describe how magnetic fields perturb the number of triplet pair product states with singlet character and how this in turn affects the singlet state kinetics. Simulations show that the magnetic field effect is very sensitive to mutual chromophore alignment, and the direction of the effect is consistent with a local ordering for rubrene molecules that participate in fission. From our analysis, the dominant fission rate is 0.5 ns–1, about 10 times slower than that observed in polycrystalline tetracene films, but we still estimate that ∼90% of the initially excited singlets undergo fission. Kinetic modeling of our fluorescence decay data and magnetic field dependence reveals that at the low laser intensities used in this experiment geminate triplet pairs do not interact with each other, and that spin–lattice relaxation between triplet sublevels is not complete on the 100 ns time scale. When both exciton fission and fusion are occurring, dynamic measurements in the presence of a magnetic field can elucidate molecular-level details of both processes.

Magnetic field dependence of singlet fission in solutions of diphenyl tetracene

“…The emission was taken from the front of the vial and passed through a dichoric filter. As noted by Bardeen and co-workers [8], the magnetic field dependence of the transient fluorescence in tetracene is expected to appear after the triplet pair states are first populated from the initial singlet exciton. The field varies the yield of singlet exciton recovery relative to the generation of independent triplets on isolated DPT molecules.…”

“…In thin films, fission has been previously characterized by transient [8,15] and continuous wave [10] optical probes. Both show a strong magnetic field dependence in the fluorescence.…”

Nanosecond pump and probe observation of bimolecular exciton effects in rubrene single crystals

“…4, transient absorption measurements have been largely interpreted as a singlet-to-triplet fission process that occurs on a time scale on the order of $10 ps, sometimes even in nominally amorphous films. [5][6][7][8] However, Piland et al 9 measured photoluminescence transients in rubrene films that they interpreted as indicative of a fission time of 2 ns. The inability to detect tell-tale quantum beats in the rubrene photoluminescence 10 led them to hypothesize an insufficient fission rate.…”

“…15 It follows that the $10 ps lifetime mentioned above can also be interpreted as the time needed to reach an equilibrium singlet-triplet superposition state, and not necessarily the time to create two independent triplet excitons, which could be much longer, similar to the nanosecond-scale decays observed in photoluminescence transients. 9 Further investigation of excited state dynamics in rubrene on the time-scale of a few nanoseconds seems to be necessary in order to gain more insights into the fission process that ultimately leads to two independent triplet excitons.…”

Wide Field Magnetic Luminescence Imaging

“…[28][29][30][31][32][33] This refers to the magnetic field-dependent photoluminescence exhibited most commonly by some organic semiconductors and certain organometallic complexes. The strength of this magnetic photoluminescence (MPL) is usually described as …”