(Invited) High-Yield Singlet Fission of Tetracene Derivatives Utilizing Organic-Inorganic Hybrid Systems for Light Energy Conversion

Abstract

Singlet fission (SF) is highly expected to improve the solar energy conversion properties by generating two excitons from one photon absorption. Among various organic compounds, acene derivatives such as tetracene and pentacene are promising for occurrence of efficient SF. Pentacene derivatives sufficiently meet the energy-level matching condition between a singlet and two triplet states: [E(S1) ≥ 2E(T1)]. Although tetracene is well-kwown as a good candidate for SF, the number of high-yield and long-lived triplet excited states through SF is extremely limited because of the relative acceleration of the reverse triplet-triplet annihilation (TTA) considering the above-mentioned energy matching condition. Consequently, the precise control of orientation and distance between two neighboring Tc units is definitely required for long-lived individual triplets through SF but not correlated triplet pair. Because the longer lifetime of the individual triplet states is useful for efficient light energy conversion.Therefore, we first demonstrate the quantitative sequential process from intramolecular SF to intermolecular two-electron transfers using 2,2′-biphenyl-bridged tetracene dimer (Tet-BP-Tet). The obtained high-yield and long-lived triplet excited states of Tet-BP-Tet by intramolecular SF (quantum yield of individual triplet states: ΦT = 175 ± 5% and lifetime of individual triplet states: τT = 0.29 ms) resulted in the quantitative intermolecular two-electron transfer process with chloranil in benzonitrile (electron transfer yield: ΦET = 173 ± 5%).1 Then, we also propose a novel supramolecular strategy utilizing the mixed self-assembled monolayers (SAMs) with two different chain lengths. Specifically, mixed tetracene-SAMs on gold nanoclusters, which are prepared by a tetracene-modified hetero-disulfide with two different chain lengths, attain high-yield SF (Φ SF: ~90%) and individual triplet yields (Φ Τ: ~160%).2 References Nakamura, S.; Sakai, H.; Nagashima, H.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T. ACS Energy Lett. 2019, 4, 26-31.Saegusa, T.; Sakai, H.; Nagashima, H.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T. J. Am. Chem. Soc. 2019, 141, 14720-14727.