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

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2022-07, Vol.1 (26), p.9758-9772
• Main Authors: Imran, Muhammad, Pang, Junhong, Zhao, Jianzhang, Li, Ming-De
• Format: Article
• Language: English
• Subjects: Atomic energy levels; Broken symmetry; Charge transfer; Fluorescence; Luminescence; Polymer films; Singlet oxygen; Solvents; Symmetry
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/d2tc01640k

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 3 CT 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. For the first time, efficient spin-orbit charge transfer intersystem crossing ( Φ Δ = 70%) is attained via the symmetry-breaking charge transfer approach in newly designed perylene orthogonal triads.