Synthesis And Application Of Novel Triplet Photosensitizers Based On Bodipy

Triplet photo sen sitizers (TPs) are compounds that can be efficiently excited to the triplet excited state which subsequently act as catalysts in photochemical reactions. TPs are widely used for TTA upconversion, photodynamic therapy, photocatalytic organic reactions and photoinduced hydrogen production from water. A good TP should exhibit strong absorption in the visible region, a high yield of intersystem crossing (ISC) and a long triplet lifetime to allow for the energy transfer. Most transition metal complexes show efficient ISC, but small molar extinction coefficients in the visible region and short-lived triplet excited states. One obstacle to development of new TPSs is the difficulty in predicting the ISC of chromophores, especially of organic compounds without any heavy atoms. To slove these problems, based on versatile fluorophore BODIPY, we design two series novel organic TPs with the introduction of ESIPT dyes HBT/HBO and spin converter C6o, respectively.New Bodipy fluorophores conjugated with HBT/HBO units were prepared, B-l-B-3show red-shifted absorption (530-580nm; ε up to50000M-1cm-1) and emission. They show small Stokes shift (～45nm) and high fluorescence quantum yields (up to36%), which are in stark contrast to HBT/HBO. On the basis of steady state and transient absorption spectra together with DFT/TDDFT theory calculation, we propose that B-1-B-3do not undergo ESIPT. However, lifetimes of the triplet excited states are up to195μs. Different from the previous report, we demonstrated that the triplet excited states are not from the cis-keto form, just locate at BODIPY unit. The compounds were used as organic triplet photosensitizers for photooxidation of1,5-dihydroxylnaphthalene, B-3is more efficient than the conventional Ir-complex([Ir(ppy)2(phen)][PF6]) triplet photosensitizer. That means B-3possess efficient ISC property and can produce efficient1O2at room temperature.By linking together the BODIPY antenna and spin converter C60through a non-π-conjugation covalent bond, ISC predictable C60-BODIPY C-1and C-2were designed. The absorption of the dyads were readily tuned to538nm (ε=61800M-1cm-1) and597nm (ε=58200M-1cm-1) by attaching different carbazole moieties to the π-core of bodipy via ethynyl bonds. Upon excitation with green or orange laser, the intramolecular singlet excited states energy transfer efficiency from antenna to C60unit were up to97.7%. The population of the C60-localized triplet excited state of the dyads upon photoexcitation was confirmed by nanosecond timeresolved transient absorption spectroscopy and spin density surface analysis.The dyads were used as triplet photosensitizers for TTA upconversion with perylene as the acceptor, green-to-blue or orange-to-blue upconversion were observed, the upconversion quantum yields were up to2.9%.Compared to transition metal complexes, organic photosensitizers are flexible and easy to prepare, also low cost and low toxicity. They have potential applications in biomedical, development of renewable energy in the future. Our study will be useful for design of new organic triplet photosensitizers.