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Intersystem crossing via charge recombination in a perylene–naphthalimide compact electron donor/acceptor dyad

In order to study the relationship between the molecular structures of compact electron donor/acceptor dyads and the spin orbit charge transfer intersystem crossing (SOCT-ISC) efficiency, we prepared perylene (Pery)-naphthalimide (NI) dyad, in which the Pery unit is the electron donor and the NI unit is the electron acceptor, and the two units adopt almost an orthogonal geometry. The photophysical properties were studied with steady state and femtosecond/nanosecond transient absorption (fs TA and ns TA) spectroscopies, time resolved electron paramagnetic resonance (TREPR) spectroscopy and DFT comptuations. A very weak charge transfer (CT) absorption band was observed, but the fluorescence of the Pery unit is strongly quenched. Upon selective excitation into the NI unit, fast intramolecular CS process (10 ps) occurred, followed by ISC and only the 3Pery* state was obversed; whereas upon selective photoexcitation into the perylene unit, an ultrafast charge separation (0.66 ps) gives the charge separated state, followed by SOCT-ISC (8 ns) to populate the 3Pery* state, moreover; prylene radical cation was also observed, presumably formed by intermolecular charge transfer. We observed the 3Pery triplet state (lifetime = 214 microseconds) with ns TA spectroscopy. Singlet oxygen quantum yield is 80%, although the potential energy curve of the torsion between the donor and acceptor is shallow. The SOCT-ISC mechanism was confirmed by TREPR spectroscopy. The dyad was used as triplet photosensitizer for generation of delayed fluorescence (luminiscence lifetime: 57.3 microseconds).