Effect of Vibronic Coupling on Correlated Triplet Pair Formation in the Singlet Fission Process of Linked Tetracene Dimers

Tetracene-based singlet fission (SF) materials show application prospects as triplet sensitizers in organic optoelectronics. SF involves internal conversion from photoexcited singlet states ¹(S₁S₀) to correlated triplet pair states ¹(T₁T₁). We derive an expression for the internal conversion rate on the basis of the Fermi golden rule with an artificial Lorentzian broadening. The internal conversion rate depends on the interstate vibronic couplings (VCs) and energy difference (ΔE_SF) between ¹(S₁S₀) and ¹(T₁T₁). Therefore, understanding the interplay between interstate VCs and ΔE_SF is necessary to reveal how the structure-property relationship affects the SF efficiency. Here, we propose a method to quantitatively analyze interstate VCs between ¹(S₁S₀) and ¹(T₁T₁). We apply this method to SF in ortho-, meta-, and para-bis(ethynyltetracenyl)benzene and identify an effect of interstate VCs on the ¹(T₁T₁) formation rate. The interstate VCs of the meta dimer are remarkably weak, which reasonably explains the experimentally obtained slow ¹(T₁T₁) formation rate. The weak VCs result from a very small overlap density between ¹(S₁S₀) and ¹(T₁T₁) of the meta dimer. Furthermore, we investigate structure-dependence of the ¹(T₁T₁) formation rate of the para dimer and find that the para dimer shows large VCs and small ΔE_SF when the rotational angle between the two tetracene units is large, which leads to the faster ¹(T₁T₁) formation rate than those of the ortho and meta dimers. The rotation of the tetracene units is the origin of the experimentally observed fast ¹(T₁T₁) formation rate of the para dimer.