Thiol-Activated Triplet–Triplet Annihilation Upconversion: Study of the Different Quenching Effect of Electron Acceptor on the Singlet and Triplet Excited States of Bodipy

Near-infrared (NIR) fluorescence-emitting small organic molecules for cancer imaging and therapy

We discuss recent advances made in the development of NIR fluorescence-emitting small organic molecules for tumor imaging and therapy.

Controlling the triplet states and their application in external stimuli-responsive triplet-triplet-annihilation photon upconversion: from the perspective of excited state photochemistry

The property of organic light-responsive materials is determined by their electronic excited states to a large extent, for instance, the radiative decay rate constants, redox potentials, and lifetimes. Tuning the excited state properties with external stimuli will lead to versatile functional materials; a representative example is the fluorescence molecular probes, in which the singlet excited states are controlled by the external stimuli, i.e., by interaction with the analytes. In comparison, controlling the triplet excited state with external stimuli has been rarely reported, although it is also crucial for the development of novel materials for targeted photodynamic therapy (PDT) reagents and phosphorescent molecular probes. The reported results show that the principles used in singlet excited state tuning are unable to be simply applied to the triplet excited state. In this review article, we summarized the recent results on controlling the triplet excited states by the external stimuli (chemical or light), and the application of the triplet state tuning in the chemical/light controllable triplet-triplet-annihilation upconversion (TTA UC). We discussed the methods for the control of the triplet states, as well as singlet excited state, for the purpose of controlling the TTA UC. Both successful and unsuccessful methods are discussed. This information is helpful for understanding the photophysical processes in which the triplet excited state is involved, and the development of novel external stimuli-responsive triplet photosensitizers.

Design of BODIPY dyes as triplet photosensitizers: electronic properties tailored for solar energy conversion, photoredox catalysis and photodynamic therapy

BODIPYs are renowned fluorescent dyes with strong and tunable absorption in the visible region, high thermal and photo-stability and exceptional fluorescence quantum yields. Transition metal complexes are the most commonly used triplet photosensitisers, but, recently, the use of organic dyes has emerged as a viable and more sustainable alternative. By proper design, BODIPY dyes have been turned from highly fluorescent labels into efficient triplet photosensitizers with strong absorption in the visible region (from green to orange). In this perspective, we report three design strategies: (i) halogenation of the dye skeleton, (ii) donor-acceptor dyads and (iii) BODIPY dimers. We compare pros and cons of these approaches in terms of optical and electrochemical properties and synthetic viability. The potential applications of these systems span from energy conversion to medicine and key examples are presented.

An Activatable Triplet Sensitizer Based on Triplet Electron Transfer and Its Application for Triplet-Triplet Annihilation Upconversion

Activatable triplet photosensitization refers to a photosentization process which can be turned on/off easily by external stimulus. Activatable triplet photosensitizations are normally achieved by interfering with the singlet excited state before the intersystem cross process (ISC), i.e., the formation process of triplet states of sensitizer. To achieve novel activatable triplet photosensitization, a disulfide-bridged porphyrin zinc(II) dyad (ZnPor-S-S-ZnPor) is prepared. Although fast ISC can be conducted in this dyad, an extremely low efficiency is obtained when employing this dyad as a triplet donor in triplet-triplet annihilation upconversion (TTA-UC) for sensitizing perylene. This is because of the presence of electron transfer from the triplet state of the porphyrin zinc(II) unit to the disulfide bond, which quickly quenches the triplet state of the porphyrin zinc(II) unit. This electron transfer process can be stopped by the cleavage of the disulfide bond in the presence of thiol, and TTA-UC efficiency can be enhanced significantly. Our result demonstrates for the first time that the disulfide bond can act as not only an easy cleavage linker but also a triplet electron acceptor. Furthermore, quenching the triplet states of sensitizer by triplet electron transfer provides an alternative protocol for designing activatable triplet sensitizers except controlling the singlet excited state before the ISC process.

A highly sensitive two-photon fluorescent probe for glutathione with near-infrared emission at 719 nm and intracellular glutathione imaging

A near-infrared turn-on two-photon fluorescent probe ST-BODIPY for glutathione-specific detection was designed and synthesized by attaching triphenylamine to BODIPY skeleton through the Knoevenagel condensation to prolong the maximum emission wavelength to the NIR region. And 2,4-dinitrobenzenesulfonyl group (DNBS), as the fluorescence quencher and thiol recognition moiety, was modified in 8 position of BODIPY. In the presence of GSH, the probe afforded an "off-on" signal response with a significant NIR fluorescence enhancement centered at 719 nm accompanying by quantum yield increased to 0.44, which was ascribed to the glutathione-induced S_NAr (aromatic substitution) reaction. Surprisingly, we found that the probe could discriminate GSH from other biothiols including Cys and Hcy upon the addition of intracellular concentrations of them. Time-dependence also demonstrated that the probe could distinguish GSH from Cys and Hcy under physiological environment. The limit of detection (LOD) for GSH was calculated as 25.46 nM from the titration experiments, which is lower than most previously reported GSH-selective probes. Under the Ti:sapphire pulsed laser's 800 nm irradiation, ST-BODIPY toward GSH generated an "off-on" signal response with a significant enhancement of fluorescence emission at 719 nm after treatment with GSH. Besides, the 2PA cross section value (σ₂) was calculated to be 410 GM, suggesting that it could not only function well as an excellent two-photon fluorescent probe for the detection of intracellular GSH, but also be applied for two-photon imaging with high sensitivity in living cells. Moreover, ST-BODIPY probe has been successfully employed for monitoring exogenous and endogenous GSH in MCF-7 cells with satisfying results, perhaps it was feasible for detecting abnormal contents of GSH in a biological system and accomplishing the goal of maintaining normal human activities.

Synthesis of a BODIPY disulfonate near-infrared fluorescence-enhanced probe with high selectivity to endogenous glutathione and two-photon fluorescent turn-on through thiol-induced SNAr substitution

A BODIPY disulfonate BODIPY-diONs with two-photon fluorescent turn-on effect was developed as fluorescence probe for selective detection of glutathione over cysteine and homocysteine. BODIPY-diONs is weakly fluorescent due to the 2,4-dinitrobenzenesulfonyl quencher group. When GSH was added, a S_NAr substitution reaction was triggered. The red emission of the BODIPY fluorophore at 675 nm was switched on, with a 27-fold emission enhancement in fluorescence intensity. The color of the solution changed from blue to green together with fluorescence appeared within 5 s. The absorbance and emission maxima of the probe BODIPY-diONs were achieved at 650 nm and 675 nm, respectively (quantum yield: 0.11). Interestingly, under the sapphire pulsed laser's 800 nm irradiation, in presence of GSH, the two-photon excited fluorescence (TPEF) of probe BODIPY-diONs was turned on, affording an OFF-ON response signal and a strong emission band at 682 nm. Furthermore, for detection of GSH, the chemodosimeter BODIPY-diONs exhibits high sensitivity and excellent anti-interference with low detection limit of 0.17 μM, and it works effectively within a wide pH range. Furthermore, the imaging studies proved that the probe BODIPY-diONs is suitable for the detection of GSH in complete physiological media.

PyrrolylBODIPYs: Syntheses, Properties, and Application as Environment-Sensitive Fluorescence Probes

Four pyrrole B-ring-functionalized pyrrolylBODIPYs and their B-ring unsubstituted analogues were synthesized from easily accessible starting 5-halo-2-formylpyrroles and were characterized by nuclear magnetic resonance, high-resolution mass spectrometry, X-ray analysis, and optical/electronic properties. In great contrast to the substitution(s) at the other two pyrrolic units, electron-donating substituent(s) at pyrrole B-ring bring significant blue shift of the absorption and emission bands. Cyclic voltammetry and density functional theory calculations indicate that this blue shift may be attributed to the increased highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels and the overall increase in the energy band gaps. These pyrrolylBODIPYs generally show intense absorption (centered at 570-624 nm) and fluorescence emission (582-654 nm) in nonpolar solvents. A gradual decrease in the fluorescence intensity was observed for these dyes with the increase in solvent dipolar moment, which combines with the red to far-red absorption/emission, rendering these pyrrolylBODIPYs potential applications as environment-sensitive fluorescence probes as demonstrated in this work for bovine serum albumin.

Synthesis and spectroscopic properties of β,β'-dibenzo-3,5,8-triaryl-BODIPYs

A series of β,β'-bicyclo-3,5-diaryl-BODIPYs were synthesized from the corresponding β,β'-bicyclo-3,5-diiodo-BODIPYs (1a,b) via Pd(0)-mediated Suzuki cross-coupling reactions in 82-92% yields. Subsequent aromatization with DDQ afforded the corresponding β,β'-dibenzo-aryl-BODIPYs, which showed red-shifted absorptions and emissions in the near-IR range. The dibenzo-appended BODIPYs showed characteristic 1H-, 13C-, 11B- and 19F-NMR shifts, and nearly planar conformations by X-ray crystallography.

The unquenched triplet excited state of the fluorescent OFF/ON Bodipy-derived molecular probe based on photo-induced electron transfer

Different from the singlet excited state (fluorescence), the triplet state of the probes is not quenched by photo-induced electron transfer.

Photo-induced electron transfer in a diamino-substituted Ru(bpy)3[PF6]2 complex and its application as a triplet photosensitizer for nitric oxide (NO)-activated triplet–triplet annihilation upconversion

The photo-induced electron transfer in the diamino-Ru(bpy)₃ complex was studied with ultrafast absorption spectroscopy and was used for nitric oxide (NO)-activated triplet–triplet annihilation upconversion.

The triplet excited state of Bodipy: formation, modulation and application

The accessing of the triplet excited state of one of the most popular fluorophores, boron-dipyrromethene (Bodipy), was summarized.

H2O2-activated triplet–triplet annihilation upconversion via modulation of the fluorescence quantum yields of the triplet acceptor and the triplet–triplet-energy-transfer efficiency

H₂O₂-activatable TTA upconversion was achieved with non-fluorescent 9,10-bis(diphenylphosphino)anthracene as a triplet acceptor/emitter, which can be oxidized to a fluorescent product by H₂O₂.