| Triplet-triplet-annihilation upconversion (TTA UC) has attracted much attention, due to its advantages of low-intensity excitation power requirement (non-coherent, a few mW cm-2 is sufficient), readily tunable excitation/emission wavelength, high upconversion quantum yields, which are ideal for potential applications in photovoltaics materials, photocatalysis, biological imaging, etc. TTA UC requires a bimolecular system consisting of a triplet photosensitizer and an emitter (acceptor). Currently, the TTA UC system is uncontroallable, i.e. not responsive to the external chemical or physical stimulies. Therefore, it is of great significance to design a controallable TTA UC system. Concerning this regard, it is assumed that the TTA UC can be switched by tuning the excited state properties of photosensitizer and receptor with external stimulilies, mainly through supramolecular photochemical and chemical modification method to tune the properties of the excited state.To achieve these objectives, we employed a few different approaches. First, a ruthenium(Ⅱ) tris(bipyridine)-rhodamine triad (Ru-1) was prepared to control the triplet-state properties with external stimuli (acid/base). The rhodamine moiety is an acid-responsive module, and the RuⅡ coordination center is responsible for triplet state formation upon photoexcitation. Through steady-state UV/Vis absorption and luminescence spectroscopy, electrochemical characterization, and nanosecond/femtosecond transient absorption spectroscopy, it was found that the dyad gives a quenched triplet metal-to-ligand charge-transfer (3MLCT) excited state (lifetime τT= 103.6 ns) in the absence of acid, owing to a photoinduced electron transfer (PET) process. In the presence of acid, the rhodamine unit transforms reversibly from the spirolactam structure into the open-amide structure; thus, the PET is inhibited, the triplet-state lifetime (5.70 μs) is prolonged, and the T1 state is relocated to the rhodamine unit. Intramolecular singlet-triplet energy transfer (STEnT,KsTEnT= 6.65 × 109 s-1) and triplet-triplet energy transfer (TTET,3MLCT→3IL, KTTET= 8.9× 106 s-1) were determined for the triad by the femtosecond and nanosecond transient absorption spectra, respectively. The dyad was used for acid-controllable triplet-triplet annihilation upconversion.Second, reversible photoswitched TTA UC was demonstrated with dithienylethenederivatives (DTE-1 and DTE-2) as the photochromic units. The TTA UC can be switched by changing the DTE structure with different energy levels of the excited states in the supramolecular system, and the UC is undisturbed by the open-form DTE but can be switched OFF upon photoirradiation of the mixture of the three components at 254 nm, i.e., by the closed form DTE. Subsequent visible light irradiation restores the TTA UC. By studying the competitive triplet-state energy transfer processes with nanosecond transient difference absorption and fluorescence spectroscopy, we confirmed that the quenching of the perylene triplet excited state by closed-form DTE is dominant among the four possible quenching channels.Third, in order to optimize the relevant energy levels of the T1 and the S1 states of triplet acceptors, as such to enhance the TTA process and to increase the TTA UC efficiency, a series of perylene-Bodipy dyads/triads with decreased S1 state energy levels were prepared as triplet energy acceptors (4A-2-4A-5). The excited state properties were studied in detail with steady state and time-resolved spectroscopies, and applied for TTA UC. In order to further study the mechanism of TTA, the synthesis of acceptor molecules Bodipy-Py and Bodipy-2Py were prepared. With steady state and time-resolved transient optical spectroscopies, we demonstrated that neither intramolecular homo-TTA or hetero-TTA is likely for the dyad or triad, i.e. no intramolecular TTA occur between the two perylene units or between the perylene and the Bodipy units. These results are fundamentally important for study of photochemistry and TTA UC. |
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