A Symmetric and Rigid Tetrahydropentalene Derivative as an Ideal Acceptor for Efficient Triplet–Triplet Annihilation-Assisted Photon Upconversion

The triplet-triplet annihilation (TTA)-assisted photon upconversion (PUC) properties of a solution system containing the newly developed acceptor di(p-anisyl)tetrahydropentalene TP-An, containing a rigidly constrained planar diene subunit, were elucidated. Due to its symmetric and rigid structure, TP-An undergoes rapid fluorescence emission and slow nonradiative processes, respectively, resulting in its unity fluorescence quantum yield and its short singlet excited state lifetime of 1.13 ns in degassed toluene. Its rigidity also affords a long triplet excited state lifetime of 654 μs, essential for efficient TTA-PUC. Moreover, the experimentally determined singlet (ES, 3.22 eV) and triplet (ET, 1.62-1.79 eV) excited state energy levels of TP-An meet the requirements for exergonic TTA (ES ≲ 2ET). Analysis of the green-to-purple TTA-PUC behavior (anti-Stokes shift: 0.89 eV) reveals that a toluene solution of TP-An and platinum octaethylporphyrin displays a quite low threshold excitation intensity (10 mW cm⁻²) and high PUC quantum yield (ca 23%).