Charge Transfer, Intersystem Crossing, and Electron Spin Dynamics in a Compact Perylenemonoimide-Phenoxazine Electron Donor–Acceptor Dyad

With phenoxazine (PXZ) as the electron donor and perylene-3,4-dicarboximide (PMI) as the electron acceptor, we prepared a compact, orthogonal electron donor–acceptor dyad (PMI–PXZ) to study the spin–orbit charge transfer-induced intersystem crossing (SOCT-ISC). A weak charge transfer (CT) absorption band, due to S0 → 1CT transition, was observed (ε = 2840 M–1 cm–1 at 554 nm, FWHM: 2850 cm–1), which is different from that of the previously reported analogue dyad with phenothiazine as the electron donor (PMI–PTZ), for which no CT absorption band was observed. A long-lived triplet state was observed (lifetime τT = 182 μs) with nanosecond transient absorption spectroscopy, and the singlet oxygen quantum yield (ΦΔ = 76%) is higher than that of the previously reported analogue dyad PMI–PTZ (ΦΔ = 57%). Ultrafast charge separation (ca. 0.14 ps) and slow charge recombination (1.4 ns) were observed with femtosecond transient absorption spectroscopy. With time-resolved electron paramagnetic resonance spectroscopy (TREPR), we confirmed the SOCT-ISC mechanism, and the electron spin polarization phase pattern of the triplet-state TREPR spectrum is (e, e, a, e, a, a), which is dramatically different from that of PMI–PTZ (a, e, a, e, a, e), indicating that the triplet-state TREPR spectrum of a specific chromophore in the electron donor–acceptor dyads is not only dependent on the geometry of the dyads but also dependent on the structure of the electron donor (or acceptor). Even one-atom variation in the donor structure may cause significant influence on the electron spin selectivity of the ISC of the electron donor–acceptor dyads.

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