Electron delocalization in one-dimensional perylenediimide nanobelts through photoinduced electron transfer

Photoinduced electron transfer (PET) of a hybrid system comprising N,N′-ditridecylperylenediimide (LPDI), which forms nanobelt structures of the form (LPDI)_n, and soluble zinc (tetra-tert-butyl)phthalocyanine (ZnTBPc) has been investigated in polar benzonitrile. The PET of a mixture system comprising N,N′-diheptadecan-9-ylperylene- diimide (BPDI) dissolved thoroughly in benzonitrile and ZnTBPc was also examined for comparison. LPDI nanobelt structures were identified using steady-state absorption and emission spectroscopies, as well as dynamic light scattering (DLS), in suspension and detected using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in the solid state. The electron paramagnetic resonance (EPR) spectrum of the radical anion of LPDI nanobelts [(LPDI)_n•-] was quite different from that of BPDI (BPDI•-) because of enhanced electron delocalization within the one-dimensional LPDI aggregates. Polar benzonitrile enables intermolecular light-induced electron transfer from the low-lying triplet state of ZnTBPc to the LPDI nanobelts through its stabilization effect on the electron-transfer species, as indicated by free energy calculations. Nanosecond transient absorption spectra displayed marked broadening of the radical anion peak of LPDI nanobelts in the near-infrared (NIR) region upon excitation, confirming the delocalization of the transferred electron within the nanostructure. Whereas both the hybrid and mixture systems have nearly the same rate constants (k_et) of PET from the PDIs to ZnTBPc, the rate of back electron transfer (k_bet) of (LPDI)_n•-/ ZnTBPc^•+ is lower than that of BPDI•-/ZnTBPc ^•+, which might result from the effect of electron delocalization within the nanobelt structure.