Radical-Enhanced Intersystem Crossing in a Bay-Substituted Perylene Bisimide-TEMPO Dyad and the Electron Spin Polarization Dynamics upon Photoexcitation*.

A 4-amino-2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was attached to the bay position of perylene-3,4 : 9,10-bis(dicarboximide) (perylenebisimide, PBI) to study the radical-enhanced intersystem crossing (REISC) and electron spin dynamics of the photo-induced high-spin states. The dyads give strong visible light absorption (ϵ=27000 M  cm at 607 nm). Attaching a TEMPO radical to the PBI unit transforms the otherwise non-radiative decay of S state (fluorescence quantum yield: Φ =2.9 %) of PBI unit to ISC (singlet oxygen quantum yield: Φ =31.8 %, Φ =1.6 %). Moreover, the REISC is more efficient as compared to the heavy atom effect-induced ISC (Φ =17.8 % for 1,8-dibromoPBI). For the dyad, ISC takes 245 ps and triplet state lifetime is 1.5 μs, much shorter than the native PBI (τ =126.6 μs). X- and Q-band time-resolved electron paramagnetic resonance spectroscopy shows that the exchange interaction in the photoexcited radical-chromophore dyad is larger than the triplet zero-field splitting (ZFS) and the difference of Zeeman energies of the radical and chromophore. The inversion of electron spin polarization from emissive to absorptive was observed and attributed to the initial completion of the quartet state population and the subsequent depopulation processes induced by the zero-field splitting.