Wenbo Yang

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.

Dual phosphorescent dinuclear transition metal complexes, and their application as triplet photosensitizers for TTA upconversion and photodynamic therapy

Two novel homo Ru(II) and Ir(III) complexes (Ru-2 and Ir-2), containing a bridging boron-dipyrromethene (BODIPY) chromophore were synthesised. The BODIPY moiety was covalently attached to the coordinated bipyridine (bpy) or phenylpyridine (ppy) via two acetylene linkers to produce bimetallic-complexes, which were employed as triplet photosensitizers. Both Ru-2 and Ir-2 absorb strongly in the visible region (λabs = 570 nm, e = 113u2006317 dm−3 mol−1 cm−1 for Ru-2 and λabs = 567 nm, e = 105u2006713 dm−3 mol−1 cm−1 for Ir-2). Due to a strong intraligand feature, and a small contribution from the metal, to the triplet state, the triplet-state lifetimes are particularly long for both complexes (1316.0 μs for Ru-2, 630.7 μs for Ir-2). High upconversion quantum yields were found (19.1% for Ru-2 and 25.5% for Ir-2). The intermolecular triplet energy transfer between the metal centres were studied using nanosecond time-resolved transient absorption spectroscopy (ΦTTET = 94% Ru-2 and ΦTTET = 86% Ir-2). Knowing the desirable photophysical properties of the complexes, both were then tested for their application in photodynamic therapy (PDT).

Near-IR Broadband-Absorbing trans-Bisphosphine Pt(II) Bisacetylide Complexes: Preparation and Study of the Photophysics

Broadband near-IR absorbing trans-bis(trialkylphosphine) Pt(II) bisacetylide binuclear complex (Pt-1) was prepared with boron-dipyrromethene (Bodipy) and styrylBodipy acetylide ligands. Pt-1 shows strong absorption bands at 731 and 503 nm. Singlet energy transfer (EnT) and efficient intersystem crossing of the central coordinated Bodipy ligand were proposed to be responsible for the efficient funneling of the excitation energy to the triplet-state manifold. Reference complexes containing only a single Bodipy ligand were prepared for comparison (with styrylBodipy ligand Pt-0 or Bodipy ligand Pt-2). The molecular structures were confirmed by single-crystal X-ray diffraction. The photophysical properties were studied with steady-state and time-resolved transient absorption spectroscopies, electrochemical characterization, and density functional theory/time-dependent density functional theory calculations. Dual fluorescence was observed for Pt-1. Singlet EnT in Pt-1 was proposed based on the fluorescence quenching/excitation spectra, and femtosecond transient absorption spectra (energy transfer rate constant kEnT = 2.2 × 10(10) s(-1)). With nanosecond transient absorption spectra, intramolecular triplet-state energy transfer in Pt-1 was proved. Gibbs free energy changes of charge separation indicate that the photoinduced intramolecular electron transfer in Pt-1 is thermodynamically prohibited. Intermolecular triplet transfer between Pt-2 and L-1 was studied with nanosecond transient absorption spectra; the EnT rate and energy transfer efficiency were determined as 3.6 × 10(4) s(-1) and 94.5%, respectively. The singlet oxygen ((1)O2) photosensitizing of Pt-1 was improved as compared to the complexes containing only a single visible-light-absorbing chromophore.

Switching of the triplet excited state of styryl 2,6-diiodo-bodipy and its application in acid-activatable singlet oxygen photosensitizing.

IodoBodipy-styrylBodipy dyads triplet photosensitizers were prepared (B-1 and B-2) which contain acid-responsive moiety. Both compounds show broadband visible light absorption, due to the resonance energy transfer (RET) between the two different visible light-harvesting Bodipy units. The photophysical properties of the dyads were studied with steady-state and nanosecond time-resolved transient absorption spectroscopy. The production of triplet excited state is switched ON or OFF by protonation/deprotonation of the amino group in the dyads. In the neutral form, the excited state is short-lived (<10 ns) and no singlet oxygen ((1)O2) photosensitizing was observed. Upon protonation, a long-lived triplet excited state was observed (τT = 3.1 μs) and the (1)O2 quantum yield (ΦΔ) is up to 73.8%. The energy levels of the components of the dyads were changed upon protonation and this energy level tuning exerts significant influence on the triplet state property of the dyad. Acid-activated shuffling of the localization of the triplet excited state between two components of a dyad was observed. Furthermore, we observed a rare example that a chromophore giving shorter absorption wavelength is acting as the singlet energy acceptor in RET. The experimental results were rationalized by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations.

Broadband Visible Light-Harvesting Naphthalenediimide (NDI) Triad: Study of the Intra-/Intermolecular Energy/Electron Transfer and the Triplet Excited State

A triad based on naphthalenediimides (NDI) was prepared to study the intersystem crossing (ISC), the fluorescence-resonance-energy-transfer (FRET), as well as the photoinduced electron transfer (PET) processes. In the triad, the 2-bromo-6-alkylaminoNDI moiety was used as singlet energy donor and the spin converter, whereas 2,6-dialkylaminoNDI was used as the singlet/triplet energy acceptor. This unique structural protocol and thus alignment of the energy levels ensures the competing ISC and FRET in the triad. The photophysical properties of the triad and the reference compounds were studied with steady-state UV-vis absorption spectra, fluorescence spectra, nanosecond transient absorption spectra, cyclic voltammetry, and DFT/TDDFT calculations. FRET was confirmed with steady-state UV-vis absorption and fluorescence spectroscopy. Intramolecular electron transfer was observed in polar solvents, demonstrated by the quenching of both the fluorescence and triplet state of the energy acceptor. Nanosecond transient absorption spectroscopy shows that the T1 state of the triad is exclusively localized on the 2,6-dialkylaminoNDI moiety in the triad upon selective photoexcitation into the energy donor, which indicates the intramolecular triplet state energy transfer. The intermolecular triplet state energy transfer between the two reference compounds was investigated with nanosecond transient absorption spectroscopy. The photophysical properties were rationalized by TDDFT calculations.

Triplet Excited State of BODIPY Accessed by Charge Recombination and Its Application in Triplet–Triplet Annihilation Upconversion

The triplet excited state properties of two BODIPY phenothiazine dyads (BDP-1 and BDP-2) with different lengths of linker and orientations of the components were studied. The triplet state formation of BODIPY chromophore was achieved via photoinduced electron transfer (PET) and charge recombination (CR). BDP-1 has a longer linker between the phenothiazine and the BODIPY chromophore than BDP-2. Moreover, the two chromophores in BDP-2 assume a more orthogonal geometry both at the ground and in the first excited state (87°) than that of BDP-1 (34-40°). The fluorescence of the BODIPY moiety was significantly quenched in the dyads. The charge separation (CS) and CR dynamics of the dyads were studied with femtosecond transient absorption spectroscopy (kCS = 2.2 × 1011 s-1 and 2 × 1012 s-1 for BDP-1 and BDP-2, respectively; kCR = 4.5 × 1010 and 1.5 × 1011 s-1 for BDP-1 and BDP-2, respectively; in acetonitrile). Formation of the triplet excited state of the BODIPY moiety was observed for both dyads upon photoexcitation, and the triplet state quantum yield depends on both the linker length and the orientation of the chromophores. Triplet state quantum yields are 13.4 and 97.5% and lifetimes are 13 and 116 μs for BDP-1 and BDP-2, respectively. The spin-orbit charge transfer (SO-CT) mechanism is proposed to be responsible for the efficient triplet state formation. The dyads were used for triplet-triplet annihilation (TTA) upconversion, showing an upconversion quantum yield up to 3.2%.

Photophysical properties of palladium/platinum tetrasulfonyl phthalocyanines and their application in triplet–triplet annihilation upconversion

Triplet photosensitizers showing strong absorption in the red/deep red spectral region, high intersystem crossing, a long-lived triplet state and a high triplet state energy level are crucial for triplet–triplet annihilation upconversion. Herein we selected two tetrasulfonyl-substituted phthalocyanine (Pc) Pt(II) and Pd(II) complexes (Pd-Pc and Pt-Pc) likely to meet the above criteria. The complexes showed prolonged triplet state lifetimes (15.9 μs and 3.03 μs) and high triplet state energy levels (1.5 eV) as compared to a Pc complex bearing electron donating groups (triplet lifetimes <3.0 μs; triplet energy levels ≈1.2 eV). Weak fluorescence was observed for Pd-Pc, whereas a fluorescence/phosphorescence dual emission feature was observed for Pt-Pc. Based on the phosphorescence emission, intermolecular triplet–triplet-energy-transfer (TTET) experiments and TD–DFT computations, the T1 state energy levels of Pd-Pc and Pt-Pc were determined as 1.25 eV and 1.5 eV, respectively. The two complexes were used for triplet–triplet-annihilation upconversion, with deep red excitation at 658 nm, upconversion with a quantum yield of 0.63% and an anti-Stokes shift of 3996 cm−1 was achieved.

Ping-Pong Energy Transfer in a Boron Dipyrromethane Containing Pt(II)-Schiff Base Complex: Synthesis, Photophysical Studies, and Anti-Stokes Shift Increase in Triplet-Triplet Annihilation Upconversion

A boron dipyrromethane (BDP)-containing Pt(II)-Schiff base complex (Pt-BDP), showing ping-pong singlet-triplet energy transfer, was synthesized, and the detailed photophysical properties were investigated using various steady-state and time-resolved transient spectroscopies. Femtosecond/nanosecond transient absorption spectroscopies demonstrated that, upon selective excitation of the BDP unit in Pt-BDP at 490 nm, Förster resonance energy transfer from the BDP unit to the Pt(II) coordination center occurred (6.7 ps), accompanied by an ultrafast intersystem crossing at the Pt(II) coordination center (<1 ps) and triplet-triplet energy transfer back to the BDP moiety (148 ps). These processes generated a triplet state localized at BDP, and the lifetime was 103.2 μs, much longer than the triplet-state lifetime of Pt-Ph (3.5 μs), a complex without the BDP moiety. Finally, Pt-BDP was used as a triplet photosensitizer for triplet-triplet annihilation (TTA) upconversion through selective excitation of the BDP unit or the Pt(II) coordination center at lower excitation energy. An upconversion quantum yield of up to 10% was observed with selective excitation of the BDP moiety, and a large anti-Stokes shift of 0.65 eV was observed upon excitation of the lower-energy band of the Pt(II) coordination center. We propose that using triplet photosensitizers with the ping-pong energy-transfer process may become a useful method for increasing the anti-Stokes shift of TTA upconversion.