Ultrafast Dynamics of Photoinduced Electron Transfer in Bay-Aryl-Substituted Perylene Diimide Derivatives.

Blends of donors and acceptors have been widely used in bulk-heterojunction solar cells to have exciton formation and charge separation by photoinduced electron transfer (PET). In this work, we have synthesized perylene diimide (PDI)-based materials having different aryl substituents at the bay positions (, , and ) to understand the excited-state dynamics of electron transfer. The detailed photophysics was studied using steady-state as well as ultrafast dynamics of the excited states in different solvents. showed tremendous effects of the solvent on the electronic properties compared with the other two derivatives. The emission quantum yield of decreases drastically in dichloromethane and other polar solvents, indicating strong electron transfer. DFT calculations showed that in the HOMO is centered mostly on the -phenylcarbazole and the LUMO is on the electron-poor PDI moieties. In addition, the energy levels of the HOMO and HOMO-1 in are estimated to be identical. The free energy change for charge separation (Δ) was calculated using electrochemical and photophysical data and found to be negative for The ground- and excited-state dipole moment ratios suggest that the excited state of (1.90) is less polar than that of (3.67), which provides an idea of the lower possibility of charge separation in and . Ultrafast photodynamics studies of , , and showed fast electron transfer only in and not in the other PDI derivatives. It was also observed that electron transfer is faster in DCM and THF than in toluene. Ultrafast dynamics studies showed the presence of an equilibrium between electron transfer and decay from the singlet excited state. Ultrafast studies also showed the features of the -phenylcarbazole cation and PDI anion, further confirming the intramolecular electron transfer in .