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

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. kISC = (23 ps)−1 − (0.5 ns)−1). The lifetime of the 3*PBI state (τT = 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 (vdd) between the radical and the chromophore improve REISC efficiency. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy demonstrates different electron exchange interactions (JTR) 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 (JTR = 0.47 cm−1) to antiferromagnetic (JTR = −0.03 cm−1 and −0.01 cm−1).