Tripler pairs (TT) localized on molecular dimers formed by singlet fission have properties that make them potentially interesting for quantum information science applications. In particular, the quintet ⁵TT₀ state is maximally entangled, addressable with microwaves, and is possible to detect optically. However, population of specific spin sublevels is highly dependent on both the relative orientations of molecules within a dimer as well as their orientation relative to an applied magnetic field (B₀). Previous theoretical work predicts that selective population of ⁵TT₀ can be observed even at room temperature when the primary molecular axes and B₀ are all aligned. Motivated by this claim, we use time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy to investigate a single crystal sample of a novel tetracenethiophene material in which all molecules share parallel molecular axes, with the crystal mounted so that molecular orientation relative to B₀ could be controlled. Resulting TR-EPR spectra exhibit signs of TT as well as unpaired triplets (T + T) populated in a manner consistent with theory, with the exception that population of the higher energy ⁵TT₊₁ and ⁵TT₊₂ appear inhibited. This observation is likely explained by the presence of a dimer site with an inter-triplet exchange coupling that leads to crossings between these two states and the ³TT manifold in the field range probed by TR-EPR. The existence of such a site is verified by the observed change in photoluminescence with varying magnetic field strength in a separate experiment. These results show how engineered single crystal samples can be utilized to better understand the spin evolution of TT pairs and control the generation of potentially useful quantum states.

Sean Parkin

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