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, RB)/acceptor (naphthalimide; NI) orthogonal dyad to attain the long-lived triplet charge-transfer ((3) CT) 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 (3) NI* state. Then the slow secondary CS (125†ns) gives the long-lived (3) CT state (0.94†μs in deaerated n-hexane) with high energy level (ca. 2.12†eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as (1) NI*→(1) CT→(3) NI*→(3) CT. 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 to previous methods for which invoking transition-metal coordination or chromophores with intrinsic ISC ability is mandatory.