Effect of molecular conformation on the efficiency of the spin orbital charge recombination-induced intersystem crossing in bianthryls

Abstract

Two bianthryl dyads, with the two units connected at either the 2- or 9- positions of the anthryl moiety, were studied to establish the relationship between orientation of the anthryl moieties and the electronic coupling and intersystem crossing (ISC) efficiency. The anthryl moieties in the two dyads adopt a close-to-orthogonal geometry, with dihedral angles of 90° (9,9′-bianthryl) and 106° (2,9′-bianthryl) at the ground state, respectively. The charge transfer (CT) emission efficiency and the fluorescence lifetimes are clearly dependent on the electronic coupling between the two anthryls, and stronger coupling lead to higher fluorescence quantum yields (34% vs. 9%) and shorter luminescence lifetimes (13.9 ns vs. 38.6 ns). The bianthryl with more orthogonal geometry shows higher singlet oxygen quantum yields ΦΔ (9,9′-bianthryl, ΦΔ = 53%) than 2,9′-bianthryl (ΦΔ = 32%). Moreover, highly solvent polarity-dependent fluorescence emission and ΦΔ were observed for the dyads (ΦΔ = 22–53%), which is different from the trend of the monomer anthracene, thus we propose the spin-orbit charge transfer ISC (SOCT-ISC) is responsible for the triplet state productions of the dyads. Interestingly, we found that inducing a heavy atom (Br) does not increase the ISC yield of anthracene. ISC in bianthryls was also confirmed with nanosecond transient absorption spectroscopy, the featured T1→Tn absorption at ca. 433 nm was observed, and the triplet state lifetime are long (9,9′-bianthryl, τT = 353 μs; 2,9′-bianthryl, τT = 493 μs, in acetonitrile).