Efficient symmetry breaking spin–orbit charge transfer-induced intersystem crossing in compact orthogonal perylene-phenothiazine or -phenoxazine triads and observation of the delayed fluorescence

Abstract

Perylene-based orthogonal triads were synthesized by attaching two identical electron donor units, i.e., phenothiazine (PTZ, Pery-2PTZ) and phenoxazine (PXZ, Pery-2PXZ), to investigate symmetry breaking spin–orbit charge transfer intersystem crossing (SOCT-ISC). The ISC efficiency was highly dependent on the solvent polarity, which was the maximum in solvents with medium polarity (singlet oxygen quantum yield Φ_Δ = 46–70%) and higher than that of the previously reported Pery-PTZ/Pery-PXZ dyads (Φ_Δ = 20–60% in DCM). Femtosecond transient absorption spectroscopy confirmed the symmetry-breaking charge transfer (SBCT) process in these compact electron donor–acceptor–donor (D–A–D) triads, where charge separation (CS) occurred in ca. 0.17–0.27 ps, followed by SOCT-ISC within 1.0–2.6 ns, which are faster than those in the previously reported analogue dyads (CS, 0.25–0.49 ps and ISC, 0.99–5.98 ns). An exceptionally long-lived triplet state was observed for the triads in DCM solution (τ_T = 359 μs), in glycerol triacetate (τ_T = 1.62–1.83 ms) and in polymer films (τ_T = 4.03–5.0 ms). The low-lying long-lived ³CT state of the triad in polar solvent was confirmed by an intermolecular photosensitizing method. Solvent-dependent, triplet–triplet annihilation (TTA)-based p-type delayed fluorescence was observed for the triads (luminescence lifetime = 114 μs under the specific experimental conditions). The triads were also used as efficient triplet photosensitizers for the generation of delayed fluorescence with solvent Green-5 as the triplet acceptor and a long luminescence lifetime up to 94.8 μs was observed.