Accessing the Triplet State in Heavy-Atom-Free Perylene Diimides

Previous studies of perylenediimides (PDIs) mostly utilized the lowest singlet excited state S-1. Generation of a triplet excited state (T-1) in PDIs is important for applications ranging from photodynamic therapy to photovoltaics; however, it remains a formidable task. Herein, we developed a heavy-atom-free strategy to prompt the T1S1 intersystem crossing (ISC) by introducing electron-donating aryl (Ar) groups at the head positions of an electron-deficient perylenediimide (PDI) core. We found that the ISC efficiency increases from 8 to 54% and then to 86% by increasing the electron-donating ability of head-substituted aryl groups from phenyl (p-PDI) to methoxyphenyl (MeO-PDI) and then to methylthioxyphenyl (MeS-PDI). By enhancing the intramolecular charge-transfer (ICT) interaction from p-PDI to MeO-PDI, and then to MeS-PDI, singlet oxygen generation via energy-transfer reactions from T-1 of PDIs to O-3(2) was demonstrated with the highest yield of up to 80%. These results provide guidelines for developing new triplet-generating PDIs and related rylene diimides for optoelectronic applications.