Radical-enhanced intersystem crossing, spin dipolar interaction and electron exchange in perylenebisimide-TEMPO dyads

Abstract 4-Amino-2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was linked to perylene-3,4:9,10-bis(dicarboximide) (PBI) at varying distances and orientations. PBI-TEMPO dyads with the radical linked at the bay-position show a charge transfer absorption band in the UV−vis absorption spectra. With increasing solvent polarity, a fluorescence quenching is observed for these dyads, whereas for a derivative with TEMPO attached at the imide-position, such polarity dependency for fluorescence spectra was not observed. Steady state and femtosecond/nanosecond time-resolved optical spectroscopy confirmed the occurrence of radical-enhanced intersystem crossing (REISC. k ISC = (23 ps) −1 − (0.5 ns) −1 ). The lifetime of the 3* PBI state ( τ T = 1.0–7.6 μs) depends on the distance and orientations between TEMPO and PBI units. The results indicate that stronger electron spin–spin dipolar interaction ( v dd ) between the radical and the chromophore improve REISC efficiency. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy demonstrates different electron exchange interactions ( J TR ) in the dyads, varying from ferromagnetic interaction corresponding to strong exchange regime to weak antiferromagnetic exchange interaction with increasing the distance between PBI and TEMPO units. Transient-nutation experiments further clarify the TREPR signals. DFT calculations indicate that changes in the dyad structure alter the exchange coupling from ferromagnetic ( J TR = 0.47 cm −1 ) to antiferromagnetic ( J TR = −0.03 cm −1 and −0.01 cm −1 ).