Enhancing the deep-red/near-infrared fluorescence of higher rylene diimides via the chalcogen-annulation strategy

Deep-red/near-infrared fluorescence is highly suitable for bioimaging owing to its ability to deeply penetrate tissues, organs, and live animals. However, developing organic fluorophores with high deep-red/near-infrared fluorescence quantum yield (Φ_FL) and fluorescent brightness remain a significant challenge owing to the energy gap law. Herein, we developed a straightforward and effective chalcogen-annulation strategy by introducing O, S and Se into the bay region of TDI and QDI fluorophores, realizing the increase of Φ_FL and fluorescent brightness up to 10 times. To our best knowledge, this study potentially stands as the pioneering instance showcasing the anti-heavy-atom effect of chalcogens, and the absolute Φ_FL (93%) and fluorescent brightness (128,200 cm⁻¹ mol⁻¹ L) of Se-TDI is among top deep-red/near-infrared organic fluorophores currently available. The femtosecond transient absorption (fs-TA) measurements show the absence of obvious changes of the excited state lifetime after the introduction of chalcogens in TDI and QDI fluorophores, indicating that intersystem crossing (ISC) can be neglected in TDI and QDI fluorophores. Theoretical calculations further reveal the chalcogen-annulation strategy increase the radiative rates and reduce the reorganization energy of several accepting modes at the ground state in TDI fluorophores, leading to the suppression of internal conversion (IC) processes. Our chalcogen-annulation strategy, which effectively increases the Φ_FL and restricts the IC processes, while remaining unaffected by the heavy-atom effect, offers novel insights and theoretical support for the design and synthesis of deep-red/near-infrared organic fluorophores with high Φ_FL and fluorescent brightness.