Excited-State Photophysical Processes in a Molecular System Containing Perylene Bisimide and Zinc Porphyrin Chromophores

Multichromophoric systems with efficient photoinduced excited-state processes are important to convert solar energy in the artificial photosynthesis. However, low molecular absorption coefficient in the near-infrared region is the limitation of power conversion efficiency in organic solar cells. It is critical to design molecules with broad absorption in the whole spectral region to better harvest solar energy, and to reveal their important multiple-step photophysical processes for the design of organic solar cells. Here, we investigate a novel compound having three chromophores, namely two near-by N,N’-bis(1-pentyl)hexyl-3,4,9,10-perylenebiscarboximide (PDI) linking to a zinc porphyrin core side by side (in the form of PDI-ZnPor-PDI), which absorbs solar energy ranging from ultraviolet (UV) to near-infrared region. The photophysical behaviors of this molecule, PDI-ZnPor-PDI, in both film and solution forms, have been investigated using steady-state and transient spectroscopic measurements. Charge-transfer species and triplet excited-state species are observed, whose excited-state evolutions are monitored using molecular vibrations as probes. These observations support the idea that PDI-ZnPor-PDI excited by the PDI unit generates the radical anion and triplet species of the PDI unit (PDI•− and 3PDI*) from photoinduced electron transfer process. Our results demonstrate the effect of film on the photophysical properties in such multichromophoric system, and are valuable for guiding designs and utilization of novel near-infrared electron donors or acceptors in organic solar cells.