Abstract
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.
J.Z. thanks the NSFC (21673031, 21761142005, 21911530095 and 21421005), the State Key Laboratory of Fine Chemicals (ZYTS201901) and the Fundamental Research Funds for the Central Universities (DUT19TD28) for financial support. S.W. thanks the SIBW/DFG for financing EPR instrumentation that is operated within the MagRes Center of the University of Freiburg. We also thank Prof. J. M. Verhoeven at University of Amsterdam for helpful discussions.