Boron dipyrromethene (Bodipy) is one of the most extensively investigated organic chromophores. Most of the investigations are focused on the singlet excited state of Bodipy, such as fluorescence. In stark contrast, the study of the triplet excited state of Bodipy is limited, but it is an emerging area, since the triplet state of Bodipy is tremendously important for several areas, such as the fundamental photochemistry study, photodynamic therapy (PDT), photocatalysis and triplet-triplet annihilation (TTA) upconversion. The recent developments in the study of the production, modulation and application of the triplet excited state of Bodipy are discussed in this review article. The formation of the triplet state of Bodipy upon photoexcitation, via the well known approach such as the heavy atom effect (including I, Br, Ru, Ir, etc.), and the new methods, such as using a spin converter (e.g. C60), charge recombination, exciton coupling and the doubly substituted excited state, are summarized. All the Bodipy-based triplet photosensitizers show strong absorption of visible or near IR light and the long-lived triplet excited state, which are important for the application of the triplet excited state in PDT or photocatalysis. Moreover, the methods for switching (or modulation) of the triplet excited state of Bodipy were discussed, such as those based on the photo-induced electron transfer (PET), by controlling the competing Förster-resonance-energy-transfer (FRET), or the intermolecular charge transfer (ICT). Controlling the triplet excited state will give functional molecules such as activatable PDT reagents or molecular devices. It is worth noting that switching of the singlet excited state and the triplet state of Bodipy may follow different principles. Application of the triplet excited state of Bodipy in PDT, hydrogen (H2) production, photoredox catalytic organic reactions and TTA upconversion were discussed. The challenges and the opportunities in these areas were briefly discussed.

2,6-diiodoBodipy-perylenebisimide (PBI) dyad and triad were prepared, with the iodoBodipy moiety as the singlet/triplet energy donor and the PBI moiety as the singlet/triplet energy acceptor. IodoBodipy undergoes intersystem crossing (ISC), but PBI is devoid of ISC, and a competition of intramolecular resonance energy transfer (RET) with ISC of the diiodoBodipy moiety is established. The photophysical properties of the compounds were studied with steady-state and femtosecond/nanosecond transient absorption and emission spectroscopy. RET and photoinduced electron transfer (PET) were confirmed. The production of the triplet state is high for the iodinated dyad and the triad (singlet oxygen quantum yield ΦΔ = 80%). The Gibbs free energy changes of the electron transfer (ΔGCS) and the energy level of the charge transfer state (CTS) were analyzed. With nanosecond transient absorption spectroscopy, we confirmed that the triplet state is localized on the PBI moiety in the iodinated dyad and the triad. An exceptionally long lived triplet excited state was observed (τT = 150 μs) for PBI. With the uniodinated reference dyad and triad, we demonstrated that the triplet state localized on the PBI moiety in the iodinated dyad and triad is not produced by charge recombination. These information are useful for the design and study of the fundamental photochemistry of multichromophore organic triplet photosensitizers.

Activatable triplet-triplet annihilation upconversion was achieved using aminomethyleneanthracene derivatives. The molecular structures of the anthracene derivatives were varied by changing the number of phenyl substituents on the anthracene core (A-1, A-2 and A-3 containing no phenyl, one and two phenyl substituents, respectively). The structural modifications tune the intersystem crossing (ISC), the fluorescence as well as the distance between the electron donor (amino group) and the fluorophore by using methylene (A-1 and A-2) or a benzyl moiety (A-3) as a linker. Triplet-triplet annihilation upconversion is mainly tuned by photoinduced electron transfer (PET). Hence, the fluorescence of A-1 and A-2 can be switched on by protonation or acetylation of the amino group, whereas A-3 gives persistent strong fluorescence. Determination of the Gibbs free energy changes indicated significantly different PET driving forces for the three compounds. The mechanism of the fluorescence switching was studied with steady state UV−vis absorption, fluorescence emission spectroscopy, nanosecond transient absorption spectroscopy and ab initio computations. We found that the PET exerts different quenching effects on the singlet and triplet excited states of the anthracene derivatives.The triplet-triplet annihilation upconversion using these compounds as triplet acceptors/emitter was studied as well, and it was found that upconversion can be switched on by inhibition of the PET through acetylation and protonation.synthesis. Indeed, a large panel of triplet photosensitizers 16−19 and acceptors have been developed for TTA upconversion. 1−5,20 TTA upconversion has already been applied in various research field, e.g., luminescence bioimaging, 21 photocatalysis 22 and photovoltaics. 23−27 However, activatable, or switchable TTA upconversion was rarely studied. Indeed, recently we developed photoswitchable TTA upconversion using a photochromic unit, e.g., dithienylethene (DTE). 28−30 However, to the very best of our knowledge, chemically-activatable TTA upconversion was never demonstrated before. Clearly, external stimuli-addressable TTA upconversion adds additional flexibility for the application of TTA upconversion, such as in super resolution fluorescence microscopy, 31 and its development is a major improvement.In this framework, we report herein a new chemical-activatable TTA upconversion with aminoanthracene derivatives as triplet acceptor/emitter (A-1, A-2 and A-3, see Scheme 1). The fluorescence of these anthracene derivatives was quenched by the photo-induced electron transfer (PET) from the amino group to the anthracene core (A-1 and A-2), 32,33 and is also modulated via intersystem crossing (ISC). Our conclusions are supported by the negative Gibbs free energy changes (∆G 0 CS ) of the PET processes 34 and by computational investigations. Interruption of the PET by acetylation of the amino group of the triplet acceptor switches on the TTA upconversion.Interestingly, we found that the singlet excited state of the anthracene ...

2,6-Diiodobodipy-styrylbodipy dyads were prepared to study the competing intersystem crossing (ISC) and the fluorescence-resonance-energy-transfer (FRET), and its effect on the photophysical property of the dyads. In the dyads, 2,6-diiodobodipy moiety was used as singlet energy donor and the spin converter for triplet state formation, whereas the styrylbodipy was used as singlet and triplet energy acceptors, thus the competition between the ISC and FRET processes is established. The photophysical properties were studied with steady-state UV-vis absorption and fluorescence spectroscopy, electrochemical characterization, and femto/nanosecond time-resolved transient absorption spectroscopies. FRET was confirmed with steady state fluorescence quenching and fluorescence excitation spectra and ultrafast transient absorption spectroscopy (kFRET = 5.0 × 10(10) s(-1)). The singlet oxygen quantum yield (ΦΔ = 0.19) of the dyad was reduced as compared with that of the reference spin converter (2,6-diiodobodipy, ΦΔ = 0.85), thus the ISC was substantially inhibited by FRET. Photoinduced intramolecular electron transfer (ET) was studied by electrochemical data and fluorescence quenching. Intermolecular triplet energy transfer was studied with nanosecond transient absorption spectroscopy as an efficient (ΦTTET = 92%) and fast process (kTTET = 5.2 × 10(4) s(-1)). These results are useful for designing organic triplet photosensitizers and for the study of the photophysical properties.

A heteroleptic bis(tributylphosphine) platinum(II)-alkynyl complex (Pt-1) showing broadband visible-light absorption was prepared. Two different visible-light-absorbing ligands, that is, ethynylated boron-dipyrromethene (BODIPY) and a functionalized naphthalene diimide (NDI) were used in the molecule. Two reference complexes, Pt-2 and Pt-3, which contain only the NDI or BODIPY ligand, respectively, were also prepared. The coordinated BODIPY ligand shows absorption at 503 nm and fluorescence at 516 nm, whereas the coordinated NDI ligand absorbs at 594 nm; the spectral overlap between the two ligands ensures intramolecular resonance energy transfer in Pt-1, with BODIPY as the singlet energy donor and NDI as the energy acceptor. The complex shows strong absorption in the region 450 nm-640 nm, with molar absorption coefficient up to 88 000 M(-1)  cm(-1) . Long-lived triplet excited states lifetimes were observed for Pt-1-Pt-3 (36.9 μs, 28.3 μs, and 818.6 μs, respectively). Singlet and triplet energy transfer processes were studied by the fluorescence/phosphorescence excitation spectra, steady-state and time-resolved UV/Vis absorption and luminescence spectra, as well as nanosecond time-resolved transient difference absorption spectra. A triplet-state equilibrium was observed for Pt-1. The complexes were used as triplet photosensitizers for triplet-triplet annihilation upconversion, with upconversion quantum yields up to 18.4 % being observed for Pt-1.

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A photoswitchable fluorescent triad based on dithienylethene and Bodipy was used as a triplet acceptor/emitter for reversible photoswitching of triplet-triplet-annihilation upconversion (with Pd(II) tetraphenyltetrabenzoporphyrin as a triplet photosensitizer), which shows green/near IR emission changes with an emission energy difference of 0.79 eV (Δλ = 268 nm).

Oblique shock waves are unavoidable in a rectangular hypersonic inlet, leading to a non-uniform flow field. While a significant body of the literature exists regarding the shock train modeling in a uniform incoming flow condition, few efforts have focused on the shock train behavior considering the influence of the shock wave boundary layer interactions. A low-order dynamic model of the shock train has been constructed with the help of the free interaction theory and a 1-D analysis approach. Experimental and numerical investigations have been carried out to evaluate the low-order model. The results show that the model has the capability of qualitatively analyzing the shock train behavior. In the cases with incident shocks, the rapid forward movement of the shock train has been observed by experiment. Besides this phenomenon was also modeled using the low-order model. Schlieren images show that when the shock train approaches the interaction zone, its behavior is characterized by oscillation and then follows a rapid forward movement with a linear increasing backpressure at 2.7 Ma. This phenomenon is analyzed theoretically based on the free interaction theory. Meanwhile with the help of the direct numerical simulation results from some existing studies, the flow structures in the interaction region and the following boundary layer also provide the evidence.