Red-to-Blue Photon Upconversion Based on BODIPY Derivative Photosensitizers

Triplet-triplet annihilation upconversion (TTA-UC) technology holds great promise for applications in bioimaging and photon energy conversion; however, its development is hindered by the scarcity of efficient, heavy-atom-free, red-light-absorbing organic sensitizers. In this work, two BODIPY dimers were systematically compared as sensitizers for red-light-excited TTA-UC. These include a single-bond-linked brominated derivative (2G) and a thiophene-fused heavy-atom-free derivative (FD). Combining density functional theory (DFT) calculations, nanosecond transient absorption spectroscopy, and other techniques, we investigated their photophysical properties, triplet state population dynamics, triplet-triplet energy transfer (TTET), and upconversion luminescence behavior. The results show that the FD/BPEA and FD/perylene systems achieve upconversion quantum yields of 7.7% and 8.5%, respectively, along with an anti-Stokes shift as high as 0.83 eV, despite FD containing no heavy atoms (Br, I). These upconversion performance parameters surpass those of the heavy-atom-containing 2G system (6.5% and 4.0%). This study reveals the regulatory mechanism of the linkage mode on the photophysical properties and triplet state dynamics of BODIPY dimers, providing a new strategy for designing efficient, heavy-atom-free, red-light-absorbing TTA-UC sensitizers.