Long-lived charge separated states in spiro-compact rhodamine–naphthalenediimide D–A–D systems: synthesis and time-resolved optical and electron paramagnetic resonance spectroscopic studies

A series of compact donor-acceptor triads featuring rhodamine (Rho) and naphthalenediimide (NDI) were synthesized using either direct conjugation or a flexible ethylene linker to investigate the charge separation of the compounds. The design enables access to long-lived triplet charge-separated states (CS) <i>via</i> intersystem crossing, offering an alternative to conventional singlet CS pathways. Steady-state absorption spectra showed no significant ground-state interactions between Rho and NDI units. However, fluorescence from the NDI moiety was markedly quenched in the triads compared to the reference compound, suggesting photoinduced electron transfer upon photoexcitation. Nanosecond transient absorption spectroscopy revealed that the ³CS state is formed upon photoexcitation of Rho-NDI-Rho in both non-polar hexane (HEX) and polar acetonitrile (ACN) solvents, with lifetimes of 0.11 µs (HEX) and 0.8 µs (ACN). The ³CS state lifetime is similiar in the case of the directly linked dyad and triads (Rho-NDI and Rho-NDI-Rho) (0.11 µs (HEX)) compared to the corresponding analogues possessing a flexible ethylene linker (Rho-CH-NDI and Rho-CH-NDI-CH-Rho). Specifically, Rho-CH-NDI exhibited CS state lifetimes of 0.83 µs in HEX and 0.05 µs in ACN, while Rho-CH-NDI-CH-Rho showed CS state lifetimes of 0.5 µs in HEX and 0.2 µs in ACN. Femtosecond transient absorption spectroscopy indicated that photoinduced charge separation occurred on an ultrafast timescale in all systems, mostly below the time resolution of the measurement (about 200 fs). Additionally, electron paramagnetic resonance (EPR) spectra of the Rho-CH-NDI system showed weak electronic coupling between the Rho and NDI units. Time-resolved EPR (TREPR) spectra confirmed the formation of ³NDI*, with zero-field splitting (ZFS) parameters |<i>D</i>| = 2150 MHz and |<i>E</i>| = 0 MHz. Furthermore, narrow signals attributed to spin-correlated radical pairs (SCRPs) were observed in the triads, with the electron exchange energy (2<i>J</i>) determined as -100 MHz.