Multiexciton quintet state populations in a rigid pyrene-bridged parallel tetracene dimer

The multiexciton quintet state, ⁵TT, generated as a singlet fission intermediate in pairs of molecular chromophores, is a promising candidate as a qubit or qudit in future quantum information science schemes. In this work, we synthesize a pyrene-bridged parallel tetracene dimer, TPT, with an optimized interchromophore coupling strength to prevent the dissociation of ⁵TT to two decorrelated triplet (T₁) states, which would contaminate the spin-state mixture. Long-lived and strongly spin-polarized pure ⁵TT state population is observed via transient absorption spectroscopy and transient/pulsed electron paramagnetic resonance spectroscopy, and its lifetime is estimated to be >35 µs, with the dephasing time (T₂) for the ⁵TT-based qubit measured to be 726 ns at 10 K. Direct relaxation from ¹TT to the ground state does diminish the overall excited state population, but the exclusive ⁵TT population at large enough persistent density for pulsed echo determination of spin coherence time is consistent with recent theoretical models that predict such behavior for strict parallel chromophore alignment and large exchange coupling.