NIR‐Absorbing B,N‐Heteroarene as Photosensitizer for High‐Performance NIR‐to‐Blue Triplet‐Triplet Annihilation Upconversion

Abstract

AbstractTriplet‐triplet annihilation upconversion (TTA‐UC) with near‐infrared (NIR) photosensitizers is highly desirable for a variety of emerging applications. However, the development of NIR‐to‐blue TTA‐UC with a large anti‐Stokes shift is extremely challenging because of the energy loss during the intersystem crossing (ISC). Here, we develop the first NIR‐absorbing B,N‐heteroarene‐based sensitizer (BNS) with multi‐resonance thermally activated delayed fluorescence (MR‐TADF) characters to achieve efficient NIR‐to‐blue TTA‐UC. The small energy gap between the singlet and triplet excited states (0.14 eV) of BNS suppresses the ISC energy loss, and its long‐delayed fluorescence lifetime (115 μs) contributes to efficient triplet energy transfer. As a result, the largest anti‐Stokes shift (1.03 eV) among all heavy‐atom‐free NIR‐activatable TTA‐UC systems is obtained with a high TTA‐UC quantum yield of 2.9 % (upper limit 50 %).