Charge Separation/Recombination, Intersystem Crossing, and Unusually Slow Intramolecular Triplet–Triplet Energy Transfer in Naphthalenediimide–Anthracene Compact Energy Donor–Acceptor Dyads

Three anthracene (An)–naphthalenediimide (NDI) compact electron donor–acceptor dyads were prepared. Femtosecond transient absorption (fs-TA) spectra show fast charge separation (ca. 0.9–1.7 ps) and relatively slow charge recombination (ca. 8–565 ps) upon photoexcitation; moreover, the 3 An state was observed for 9-An-NDI, whereas the final state is 3 NDI for both 9-An-Ph-NDI and 2-An-Ph-NDI, which have an intervening phenyl linker between the An and NDI units. Nanosecond transient absorption (ns-TA) spectra indicate that the lowest triplet state of all the dyads is 3 An, with triplet lifetimes of 139–354 μs. An unusually slow intramolecular triplet–triplet energy transfer (TTET) was observed for 9-An-Ph-NDI and 2-An-Ph-NDI (32–85 ns). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy confirms that the intersystem crossing (ISC) mechanism is spin orbit charge transfer ISC (SOCT-ISC) for all the dyads; for 9-An-NDI, only the 3 An state was observed, while for the other two dyads, both 3 NDI and 3 An states were observed, with their relative population changing with increasing delay time, which supports TTET.