Long-Lived Charge Transfer State Induced by Spin-Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in a Compact Spiro Electron Donor/Acceptor Dyad.

We prepared conceptually novel, fully rigid, spiro compact electron donor (Rhodamine B, lactam form)/acceptor (naphthalimide) orthogonal dyad to attain the long-lived triplet charge transfer (3CT) state, based on the electron spin control using spin-orbit charge transfer intersystem crossing (SOCT-ISC). Transient absorption (TA) spectra indicate the first charge separation (CS) takes place within 2.5 ps, subsequent SOCT-ISC takes 8 ns to produce the 3NI* state. Then the slow secondary CS (125 ns) gives the long-lived 3CT state (0.94 microseconds in deaerated n-hexane) with high energy level (ca. 2.12 eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as 1NI*-->1CT-->3NI*-->3CT. With time-resolved electron paramagnetic resonance (TREPR) spectra, an EEEAAA electron-spin polarization pattern was observed for the naphthalimide-localized triplet state. Our spiro compact dyad structure and the electron spin-control approach is different as compared to previous methods for which invoking transition-metal coordination or chromophores with intrinsic ISC ability is mandatory. This new method of accessing long-lived 3CT states is useful for artificial photosynthesis, photovoltaics, photocatalysis and fundamental photochemistry studies.