Organic photosensitizers possessing efficient intersystem crossing (ISC) and forming long-living triplet excited states, play a crucial role in a number of applications. A common approach in the design of such dyes relies on the introduction of heavy atoms (e.g. transition metals or halogens) into the structure, which promote ISC via spin–orbit coupling interaction. In recent years, alternative methods to enhance ISC have been actively studied. Among those, the generation of triplet excited states through photoinduced electron transfer (PET) in heavy-atom-free molecules has attracted particular attention because it allows for the development of photosensitizers with programmed triplet state and fluorescence quantum yields. Due to their synthetic accessibility and tunability of optical properties, boron dipyrromethenes (BODIPYs) are so far the most perspective class of photosensitizers operating via this mechanism. This article reviews recently reported heavy-atom-free BODIPY donor–acceptor dyads and dimers which produce long-living triplet excited states and generate singlet oxygen. Structural factors which affect PET and concomitant triplet state formation in these molecules are discussed and the reported data on triplet state yields and singlet oxygen generation quantum yields in various solvents are summarized. Finally, examples of recent applications of these systems are highlighted.

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Heavy-atom-free BODIPY photosensitizers with intersystem crossing mediated by intramolecular photoinduced electron transfer

Photo-induced charge separation (CS) and charge recombination (CR) are crucial for molecular systems used in artificial photosynthesis, photovoltaics, and photocatalysis, and these processes are also important for fundamental photochemical studies. This review article focuses on the recent developments in CS/CR, long-lived charge transfer (CT) state formation, and CR-induced intersystem crossing (ISC) in organic molecular systems, ranging from discussions on the molecular structural features dictating the kinetics and thermodynamics of CS/CR and electron spin transport, as well as the spectroscopies used for the characterization of the CS/CR and the CR-induced ISC. Emphases are put on the molecular design rationales, from the point of view of organic chemists, to achieve fast and efficient CS but slow CR, as such to access the long-lived CT state. The roles of the electron spin in controlling the CS, CR and formation of the long-lived CT state are discussed with exemplars. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy for characterization of the charge and electron spin transport in the CS, CR and ISC processes is introduced.

Charge separation, charge recombination, long-lived charge transfer state formation and intersystem crossing in organic electron donor/acceptor dyads

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.

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

Theranostics that combines both diagnosis and therapy into a single platform has recently emerged as a promising biomedical approach for cancer treatment; however, the development of efficient theranostic agents with excellent optical properties remains a challenge. Here, we report novel mitochondria-targeting BODIPY photosensitizers (R-BODs) that possess considerable singlet oxygen generation capabilities and good fluorescence properties for imaging-guided photodynamic therapy (PDT). The incorporation of sulfur atoms into the π-conjugated skeleton of BODIPY along with the introduction of different functional groups at the meso-position of the BODIPY core is essential for tuning the photophysical and photosensitizing properties. Notably, the MeOPh-substituted thiophene-fused BODIPY (MeO-BOD, R = p-methoxyphenyl) displayed the highest singlet oxygen generation capability (ΦΔ ≈ 0.85 in air-saturated acetonitrile) and a moderate fluorescence quantum yield (Φf = 17.11). Furthermore, MeO-BOD showed good biocompatibility, low dark toxicity and superior fluorescence imaging properties in living cells. More importantly, the PDT efficacy of mitochondria-specific anchoring of MeO-BOD was remarkably amplified with an extremely low half-maximal inhibitory concentration (IC50) value of 95 nM. We believe that the incorporation of an electron-donating group at the meso-position of the thiophene-fused BODIPY platform may be an effective approach for developing theranostic agents for precision cancer therapy.

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Fine-tuning the electronic structure of heavy-atom-free BODIPY photosensitizers for fluorescence imaging and mitochondria-targeted photodynamic therapy

A series of Bodipy dimers with orthogonal conformation were prepared. The photophysical properties were studied with steady-state and time-resolved transient spectroscopies. We found the triplet-state quantum yield is highly dependent on the solvent polarity in the orthogonally linked symmetric Bodipy dimers, and the intersystem crossing (ISC) is efficient in solvents with moderate polarity. The photoinduced symmetry-breaking charge transfer (SBCT) in polar solvents was confirmed by femtosecond transient absorption spectroscopy, with the charge separation (CS) kinetics on the order of a few picoseconds and the charge recombination (CR) process occurring on the nanosecond time scale in dichloromethane. These observations are supported by the calculation of the charge separated state (CSS) energy levels, which are high in nonpolar solvents, and lower in polar solvents, thus the CR-induced ISC has the largest driven force in solvents with moderate polarity. These results clarify the mechanism of SOCT-ISC in ...

A Revisit to the Orthogonal Bodipy Dimers: Experimental Evidence for the Symmetry Breaking Charge Transfer-Induced Intersystem Crossing

Modifying the meso-phenyl with electron donating amino groups strongly enhances BODIPY's ability as good singlet oxygen photosensitizer

Heavy-atom-free charge transfer photosensitizers: Tuning the efficiency of BODIPY in singlet oxygen generation via intramolecular electron donor-acceptor interaction

Attaching electron donating groups on the meso -phenyl and meso -naphthyl make aryl substituted BODIPYs act as good photosensitizer for singlet oxygen formation

Xian-Fu Zhang

Papers

Modifying the meso-phenyl with electron donating amino groups strongly enhances BODIPY's ability as good singlet oxygen photosensitizer

Heavy-atom-free charge transfer photosensitizers: Tuning the efficiency of BODIPY in singlet oxygen generation via intramolecular electron donor-acceptor interaction

Attaching electron donating groups on the meso -phenyl and meso -naphthyl make aryl substituted BODIPYs act as good photosensitizer for singlet oxygen formation