The properties and formation mechanisms of the triplet state have been widely investigated since they are crucial intermediates in photo functional devices. Specifically, helical PDI dimers, horizontal expanded π-conjugated derivatives of PDI, have shown outstanding performance as electron acceptors in enhancing the performance of photovoltaics. Therefore, the exploration of triplet generation in helical PDI dimers plays a crucial role in understanding the mechanisms and excavating their further application. We make use of Se-annulation to induce intersystem crossing (ISC) in helical PDI dimers and further explore the triplet evolution process systematically as the number of Se atoms increases by transient absorption spectroscopy and the hole-electron analysis method. It shows that the twisted molecular conformation has paved the way for potential ISC in a parent molecule PDI2. The incorporation of Se atoms can result in evident promotion in the efficiency of ISC (ϕTPDI2-2Se = 96.9%) compared to the parent molecule PDI2 (ϕTPDI2 = 26.5%), indicating that chalcogen-annulation is also an efficient strategy in a π-extended system. Our results provide useful insights for understanding the triplet evolution process, which can help broaden the application of the π-extended PDI system into high-performance photovoltaics.