Xiaoneng Cui

Reversible Photoswitching of Triplet–Triplet Annihilation Upconversion Using Dithienylethene Photochromic Switches

Reversible photoswitched triplet-triplet annihilation upconversion (TTA UC) was demonstrated with dithienylethene (DTE) derivatives as the photochromic units, 2,6-diiodoBodipy as the triplet photosensitizer, and perylene as the triplet acceptor/emitter. The TTA 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 time-resolved 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 processes.

Observation of the room temperature phosphorescence of Bodipy in visible light-harvesting Ru(II) polyimine complexes and application as triplet photosensitizers for triplet–triplet-annihilation upconversion and photocatalytic oxidation

Two Ru(II) polyimine complexes containing a boron-dipyrromethene (Bodipy) chromophore were prepared. The two complexes are different in the linker which connects the Bodipy part and the Ru(II) coordination centre. The Bodipy core and the Ru(II) centre are in π-conjugation in Ru-1, whereas in Ru-2 the Bodipy part is linked in a non-conjugated way to the Ru(II) centre. Ru(bpy)3[PF6]2 (Ru-3) was used as a reference complex. Both Ru-1 and Ru-2 show strong absorption in the visible region (e = 65 200 M−1 cm−1 at 528 nm for Ru-1 and e = 76 700 M−1 cm−1 at 499 nm for Ru-2). The fluorescence of the Bodipy ligands was almost completely quenched in Ru-1 and Ru-2. Ru-1 shows room temperature phosphorescence of the Bodipy chromophore, as well as the residual fluorescence of the Bodipy ligand. Ru-2 shows only the residual fluorescence of the Bodipy ligand. A long-lived Bodipy-localized triplet excited state was observed for both Ru-1 and Ru-2 upon visible light excitation (τT is up to 279.7 μs, the longest T1 state lifetime observed for the Bodipy moiety in the transition metal complex). Application of the complexes in triplet–triplet-annihilation upconversion and singlet oxygen (1O2)-mediated photo-oxidation proved that Ru-1 is more efficient (e.g. singlet oxygen quantum yield ΦΔ = 0.93) as a triplet photosensitizer than Ru-2 (ΦΔ = 0.64). Therefore, direct connection of the π-core of the Bodipy chromophore to the coordination centre, i.e. by establishing π-conjugation between the visible light-harvesting chromophore and the metal coordination centre is essential to enhance the effective visible light-harvesting of the Ru(II) complexes.

Enhanced photooxidation sensitizers: the first examples of cyclometalated pyrene complexes of iridium(III).

The iridium(III) cyclometalation of alkylated pyrene-benzimidazole ligands proceeds in an unprecedented manner. The resultant complexes display remarkably enhanced photooxidation capabilities using 1,5-dihydroxynaphthalene as a substrate.

Photoswitching of the Triplet Excited State of DiiodoBodipy-Dithienylethene Triads and Application in Photo-Controllable Triplet-Triplet Annihilation Upconversion

Dithienylethene (DTE)-2,6-diiodoBodipy triads were prepared with the aim to photoswitch the triplet excited state of the 2,6-diiodoBodipy moiety. Bodipy was selected due to its low T1 state energy level to avoid sensitized photocyclization of DTE, which is very often encountered in DTE photoswitches, so that the photochemistry of DTE and the organic chromophore can be addressed independently. This is the first time that DTE was covalently connected with an organic triplet photosensitizer. For the triad with DTE-o structure, selective photoexcitation into the diiodoBodipy part did not initiate photocyclization of DTE-o. Upon photoirradiation at 254 nm, thus the DTE-o → DTE-c transformation, the intersystem crossing (ISC) of 2,6-diiodoBodipy moiety was competed by the photoactivated resonance energy transfer (RET), with Bodipy as the intramolecular energy donor and DTE-c as energy acceptor. The fluorescence of Bodipy was quenched and the triplet state lifetime of Bodipy was reduced from 105.1 to 40.9 μs. The photoreversion is O2-independent, but can be greatly accelerated upon selective photoexcitation into the diiodoBodipy absorption band (at 535 nm). We concluded that ISC is not outcompeted by RET. The photoswitching of the triplet state was transduced to the singlet oxygen photosensitizing, as well as triplet-triplet annihilation upconversion.

Perylene-derived triplet acceptors with optimized excited state energy levels for triplet-triplet annihilation assisted upconversion.

A series of perylene derivatives are prepared as triplet energy acceptors for triplet-triplet annihilation (TTA) assisted upconversion. The aim is to optimize the energy levels of the T1 and S1 states of the triplet acceptors, so that the prerequisite for TTA (2E(T1) > E(S1)) can be better satisfied, and eventually to increase the upconversion efficiency. Tuning of the energy levels of the excited states of the triplet acceptors is realized either by attaching aryl groups to perylene (via single or triple carbon-carbon bonds), or by assembling a perylene-BODIPY dyad, in which the components present complementary S1 and T1 state energy levels. The S1 state energy levels of the perylene derivatives are generally decreased compared to perylene. The anti-Stokes shift, TTA, and upconversion efficiencies of the new triplet acceptors are improved with respect to the perylene hallmark. For the perylene-BODIPY dyads, the fluorescence emission was substantially quenched in polar solvents. Moreover, we found that extension of the π-conjugation of BODIPY energy donor significantly reduces the energy level of the S1 state. Low S1 state energy level and high T1 state energy level are beneficial for triplet photosensitizers.

DiiodoBodipy-Perylenebisimide Dyad/Triad: Preparation and Study of the Intramolecular and Intermolecular Electron/Energy Transfer

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.

Switching of the Triplet Triplet-Annihilation Upconversion with Photoresponsive Triplet Energy Acceptor: Photocontrollable Singlet/Triplet Energy Transfer and Electron Transfer

A photoswitchable fluorescent triad based on two 9,10-diphenylanthracene (DPA) and one dithienylethene (DTE) moiety is prepared for photoswitching of triplet-triplet annihilation upconversion. The DPA and DTE moieties in the triad were connected via Click reaction. The DPA unit in the triad was used as the triplet energy acceptor and upconverted fluorescence emitter. The fluorescence of the triad is switched ON with the DTE moiety in open form [DTE-(o)] (upconversion quantum yield ΦUC = 1.2%). Upon UV irradiation, photocyclization of the DTE-(o) moiety produces the closed form [DTE-(c)], as a result the fluorescence of DPA moiety was switched off (ΦUC is negligible). Three different mechanisms are responsible for the upconverted fluorescence photoswitching effect (i.e., the photoactivated fluorescence resonance energy transfer, the intramolecular electron transfer, as well as the photoactivated intermolecular triplet energy transfer between the photosensitizer and DTE-(c) moiety). Previously, the photoswitching of TTA upconversion was accomplished with only one mechanism (i.e., the triplet state quenching of the photosensitizer by DTE-(c) via either the intermolecular or intramolecular energy transfer). The photophysical processes involved in the photochromism and photoswitching of TTA upconversion were studied with steady-state UV-vis absorption and fluorescence emission spectroscopies, nanosecond transient absorption spectroscopy, electrochemical characterization, and DFT/TDDFT calculations.

Maximizing the thiol-activated photodynamic and fluorescence imaging functionalities of theranostic reagents by modularization of Bodipy-based dyad triplet photosensitizers

To maximize both the activatable singlet oxygen (1O2) production and fluorescence of theranostic photodynamic (PDT) reagents, herein we propose a modularized molecular structural profile, i.e. the intersystem crossing (ISC) and fluorescence functionalities were accomplished with different modules in a dyad, thus enabling the activated 1O2 production yield (ΦΔ, PDT) and the fluorescence yield (ΦF) to both approach 100%. The PDT and the fluorescence were caged with a thiol-cleavable disulfide bond (-S-S-) linker and an electron trap (2,4-dinitrobenzenesulfide, DNBS). This new molecular structural profile is different from that of conventional theranostic PDT reagents, which are based on a single chromophore for both PDT and fluorescence; thus, the limitation of ΦΔ + ΦF = 100% exists for only half of our new molecular profile. To this end, six Bodipy dyads were prepared. The photophysical properties of the dyads were studied with steady state absorption, fluorescence and nanosecond transient absorption spectroscopy. The dyads show weak PDT and luminescence, due to the caging effect. In the presence of thiols (GSH or Cys), cleavage of the disulfide linker and DNBS occurs, and the PDT and fluorescence modules are activated simultaneously (ΦF: 1.3% → 47.6%; ΦΔ: 16.7% → 71.5%). These results are useful in designing activatable PDT/fluorescence imaging theranostic reagents.

Thiol-activated triplet-triplet annihilation upconversion: study of the different quenching effect of electron acceptor on the singlet and triplet excited states of Bodipy.

Thiol-activated triplet-triplet annihilation (TTA) upconversion was studied with two different approaches, i.e., with 2,4-dinitrobenzenenesulfonyl (DNBS)-caged diiodoBodipy triplet photosensitizers (perylene as the triplet acceptor/emitter of the upconversion) and DNBS-caged Bodipy fluorophore as the triplet acceptor/emitter (PdTPTBP as the triplet photosensitizer, TPTBP = tetraphenyltetrabenzoporphyrin). The photophysical processes were studied with steady-state UV-vis absorption spectroscopy, fluorescence spectroscopy, electrochemical characterization, nanosecond transient absorption spectroscopy, and DFT/TDDFT computations. DNBS-caged triplet photosensitizer shows a shorter triplet state lifetime (24.7 μs) than the uncaged triplet photosensitizer (86.0 μs), and the quenching effect is due to photoinduced electron transfer (PET). TTA upconversion was enhanced upon cleavage of the DNBS moiety by thiols. On the other hand, the DNBS-caged Bodipy shows no fluorescence, but the uncaged fluorophore shows strong fluorescence; thus, TTA upconversion is able to be enhanced with the uncaged fluorophore as the triplet energy acceptor/emitter. The results indicate that the DNBS moiety exerts a significant quenching effect on the singlet excited state of Bodipy, but the quenching on the triplet excited state is much weaker. Calculation of the Gibbs free energy changes of the photoinduced electron transfer indicates that the singlet state gives a larger driving force for the PET process than the triplet state.

Application of singlet energy transfer in triplet state formation: broadband visible light-absorbing triplet photosensitizers, molecular structure design, related photophysics and applications

Conventional triplet photosensitizers usually contain a single visible light-harvesting chromophore, which is responsible for the dual-functionality of light-harvesting and intersystem crossing (ISC). These profiles have a few disadvantages, such as a single absorption band in the visible spectral range, low efficiency of harvesting broadband visible light (e.g., solar light), and difficulty in designing new triplet photosensitizers because the relationship between molecular structure and ISC is unclear. To address these challenges, the application of the Forster resonance energy transfer (FRET) and spin converter can lead to a new molecular structure motif for triplet photosensitizers to attain the broadband visible light-absorption, as well as disintegrated functionality of visible light-harvesting and ISC. This Review article summarizes the triplet photosensitizers showing broadband visible light absorption, including the molecular design rationales, the photophysical processes involved in these photosensitizers, such as the FRET, ISC, and the photo-induced electron transfer (PET), studied with nanosecond and femtosecond transient absorption spectroscopies. The application of triplet photosensitizers in photoredox catalytic organic reactions and triplet–triplet annihilation upconversion are also discussed. We summarized the molecular structure–property relationship of these new photosensitizers, as well as the challenges in this emerging area.