Kejing Xu

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

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.

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.

Diiodobodipy-styrylbodipy Dyads: Preparation and Study of the Intersystem Crossing and Fluorescence Resonance Energy Transfer

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.

Broadband Visible-Light-Harvesting trans-Bis(alkylphosphine) Platinum(II)-Alkynyl Complexes with Singlet Energy Transfer between BODIPY and Naphthalene Diimide Ligands

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.

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.

DiiodoBodipy-Rhodamine Dyads: Preparation and Study of the Acid-Activatable Competing Intersystem Crossing and Energy Transfer Processes

Iodo-bodipy/rhodamine dyads with cyanuric chloride linker were prepared with the goal of achieving pH switching of the triplet excited state formation. The pH switching takes advantage of the acid-activated reversible cyclic lactam↔opened amide transformation of the rhodamine unit and the fluorescence resonance energy transfer (FRET). The photophysical properties of the dyads were studied with steady-state and femtosecond/nanosecond time-resolved transient absorption spectroscopies, electrochemical methods, as well as TD-DFT calculations. Our results show that the model dyad is an efficient triplet state generator under neutral condition, when the rhodamine unit adopts the closed form. The triplet generation occurs at the iodo-bodipy moiety and the triplet state is long-lived, with a lifetime of 51.7 μs. In the presence of the acid, the rhodamine unit adopts an opened amide form, and in this case, the efficient FRET occurs from iodo-bodipy to the rhodamine moiety. The FRET is much faster (τFRET = 81 ps) than the intersystem crossing of iodo-bodipy (τISC = 178 ps), thus suppressing the triplet generation is assumed. However, we found that the additional energy transfer occurs at the longer timescale, which eventually converts the rhodamine-based S1 state to the T1 state localized on the iodo-bodipy unit.

Highly efficient near IR photosensitizers based-on Ir–C bonded porphyrin-aza-BODIPY conjugates

Two hybrid organic–metal complexes based-on Ir–C bonded porphyrin-aza-BODIPY conjugates have been facilely synthesized through group 9 iridium porphyrin with an aza-BODIPY axial ligand. The conjugates combine the advantages of the near-IR (NIR) absorption of the aza-BODPY dye (>700 nm) with the efficient singlet oxygen production of iridium porphyrin (ΦΔ = 79–85%). The experimental results demonstrate that the axial ligand of aza-BODIPY largely affects the optical and photophysical properties of the conjugates due to the strong interaction between 5d orbitals of the Ir(III) ion and the aza-BODIPY π-system in the ground state, which differs considerably from previous reports.

Rational Design of Emissive NIR-Absorbing Chromophores: Rh(III) Porphyrin-Aza-BODIPY Conjugates with Orthogonal Metal-Carbon Bonds.

The facile synthesis of Group 9 Rh(III) porphyrin-aza-BODIPY conjugates that are linked through an orthogonal Rh-C(aryl) bond is reported. The conjugates combine the advantages of the near-IR (NIR) absorption and intense fluorescence of aza-BODIPY dyes with the long-lived triplet states of transition metal rhodium porphyrins. Only one emission peak centered at about 720 nm is observed, irrespective of the excitation wavelength, demonstrating that the conjugates act as unique molecules rather than as dyads. The generation of a locally excited (LE) state with intramolecular charge-transfer (ICT) character has been demonstrated by solvatochromic effects in the photophysical properties, singlet oxygen quantum yields in polar solvents, and by the results of density functional theory (DFT) calculations. In nonpolar solvents, the Rh(III) conjugates exhibit strong aza-BODIPY-centered fluorescence at around 720 nm (ΦF =17-34 %), and negligible singlet oxygen generation. In polar solvents, enhancements of the singlet-oxygen quantum yield (ΦΔ =19-27 %, λex =690 nm) have been observed. Nanosecond pulsed time-resolved absorption spectroscopy confirms that relatively long-lived triplet excited states are formed. The synthetic methodology outlined herein provides a useful strategy for the assembly of functional materials that are highly desirable for a wide range of applications in material science and biomedical fields.