Synthesis of 9,10-Diheterocyclicanthracenes and Performance Correlations in Triplet-Triplet Annihilation Upconversion

Low-power photon upconversion (UC) can convert low-energy photons to high-energy photons based on triplet-triplet annihilation (TTA) mechanism, which invloves a bimolecular system containing a sensitizer and an emitter (acceptor). Firstly, the sensitizer harvests the excitation energy toward its triplet excited state with intersystem crossing (ISC), then the triplet energy of the sensitizer is transferred to the acceptor (i.e., triplet-triplet energy transfer, T). Lastly, two excited triplet acceptors would annihilate (i.e., triplet-triplet annihilation, TTA) while one of them irradiates the high-energy light (upconversion) from the excited state to the ground state. Since the processes mentioned above, including ISC, TTT And TTA, are transition allowed, the TTA-UC can be achieved at ultra-low excitation power that is less than the terrestrial solar radiation (ca. 100 mW.cm(-3)), which shows wide spread practical applications such as in solar cells. Compared to the development of triplet sensitizers, little attention has been paid to the development of triplet acceptors. Here, two new compounds: 9,10-Di(3-furan)anthracence (DFA) and 9, 10-di(3-thiophene)anthracence (DTA) were synthesized as the triplet acceptor (emitter) that can doped with palladium(II) tetraphenylporphyrin derivatives (PdTPPMe or PdTPPCOOH) to obtain the triplet sensitizer/acceptor bimolecular system. Under the low-power excitation of the diode laser (532 nm, 70 mW.cm(-2)), these bimolecular systems could emit strong green-to-blue upconversion with the efficiencies (Phi(UC)) in the order of DFA/PdTPPCOOH (10.11%)>DTA/PdTP-PCOOH (7.20%)>DFA/PdTPPMe (6.31%)>DFA/PdTPPMe (5.45%). The obtained results have shown that large triplet triplet quenching constant (k(Q)), high prompt fluorescence quantum (Phi(f)) and fast delayed fluorescence (tau(DF)) are responsible for effective upconversion efficiency (Phi(UC)). Moreover, it was found that oxygen-containing 9, 10-di(3-furan)anthracence (DFA) and PdTPPCOOH within the mixed alcohol media can effectively interact each other based on the hydrogen bond (H-bond) and resulting larger triplet-triplet quenching constant (k(Q))), which have contribution to enhancement the upconversion efficiency. With the help of Pt/WO3, photo-catalyst, the conversion from coumarin to 7-hydroxycoumarin was successfully achieved under the irradiation of our green-to-blue upconversion produced with bimolecular DFA/PdTPPCOOH in alcohol media.