Intersystem Crossing, Photo‐Induced Charge Separation and Regioisomer‐Specific Excited State Dynamics in Fully Rigid Spiro Rhodammine‐Naphthalene/Anthraquinone Electron Donor‐Acceptor Dyads

We prepared a series fully rigid spiro electron donor-acceptor orthogonal dyads, with closed form of rhodamine (Rho) as electron donor and naphthalene (Np)/anthraquinone (AQ) as electron acceptor, to access the long-lived triplet charge separation (³CS) state, via the electron spin control method. We found strong dependency of the photophysical property of the dyads on the amino substitution positions of the Np chromophores in the dyads 1,8-DaNp-Rho and 2,3-DaNp-Rho. Nanosecond transient absorption (ns-TA) spectra show the population of the ³LE state (lifetime: 47 μs) for 2,3-DaNp-Rho, however, long-lived ³CS state was observed (τ_CS=0.62 μs) for AQ-Rho, with a CS quantum yield of Φ_CS=58 %. Based on femtosecond transient absorption (fs-TA) spectra, spin orbit charge transfer ISC (SOCT-ISC) is proposed to be responsible for the formation of the triplet states. Time-resolved electron paramagnetic resonance (TREPR) spectra of AQ-Rho indicate the presence of two states, a ³LE state with zero field splitting (ZFS) D parameter of 1400 MHz and E parameter of -410 MHz, formed via radical pair ISC (RP-ISC) and SOCT-ISC mechanism; and a ³CS state with the electron spin-spin interaction in the regime of spin-correlated radical pair (SCRP).