Jifu Sun

Triplet photosensitizers: from molecular design to applications

Triplet photosensitizers (PSs) are compounds that can be efficiently excited to the triplet excited state which subsequently act as catalysts in photochemical reactions. The name is originally derived from compounds that were used to transfer the triplet energy to other compounds that have only a small intrinsic triplet state yield. Triplet PSs are not only used for triplet energy transfer, but also for photocatalytic organic reactions, photodynamic therapy (PDT), photoinduced hydrogen production from water and triplet-triplet annihilation (TTA) upconversion. A good PS should exhibit strong absorption of the excitation light, a high yield of intersystem crossing (ISC) for efficient production of the triplet state, and a long triplet lifetime to allow for the reaction with a reactant molecule. Most transition metal complexes show efficient ISC, but small molar absorption coefficients in the visible spectral region and short-lived triplet excited states, which make them unsuitable as triplet PSs. One obstacle to the development of new triplet PSs is the difficulty in predicting the ISC of chromophores, especially of organic compounds without any heavy atoms. This review article summarizes some molecular design rationales for triplet PSs, based on the molecular structural factors that facilitate ISC. The design of transition metal complexes with large molar absorption coefficients in the visible spectral region and long-lived triplet excited states is presented. A new method of using a spin converter to construct heavy atom-free organic triplet PSs is discussed, with which ISC becomes predictable, C60 being an example. To enhance the performance of triplet PSs, energy funneling based triplet PSs are proposed, which show broadband absorption in the visible region. Applications of triplet PSs in photocatalytic organic reactions, hydrogen production, triplet-triplet annihilation upconversion and luminescent oxygen sensing are briefly introduced.

Transition metal complexes with strong absorption of visible light and long-lived triplet excited states: from molecular design to applications

Transition metal complexes of Ru(II), Pt(II) and Ir(III) with strong absorption of visible light and long-lived T1 excited states were summarized. A design rationale of these complexes, i.e. direct metalation of organic chromophore, was proposed. Alternatively an organic chromophore can be dangled on the peripheral moiety of the coordination center. In both cases the long-lived intraligand triplet excited state (3IL) can be accessed. However, the 3IL excited state is usually emissive for the former case and it is very often non-emissive for the latter case. Two methods used for study of the long-lived triplet excited state, i.e. the time-resolved transient difference absorption spectroscopy and the spin density analysis, are briefly introduced. Preliminary applications of the complexes in luminescent O2 sensing and triplet–triplet annihilation (TTA) upconversions were discussed.

Long-Lived Room-Temperature Near-IR Phosphorescence of BODIPY in a Visible-Light-Harvesting N^C^N PtII–Acetylide Complex with a Directly Metalated BODIPY Chromophore†

Room-temperature long-lived near-IR phosphorescence of boron-dipyrromethene (BODIPY) was observed (λ(em) = 770 nm, Φ(P) = 3.5 %, τ(P) = 128.4 μs). Our molecular-design strategy is to attach Pt(II) coordination centers directly onto the BODIPY π-core using acetylide bonds, rather than on the periphery of the BODIPY core, thus maximizing the heavy-atom effect of Pt(II). In this case, the intersystem crossing (ISC) is facilitated and the radiative decay of the T(1) excited state of BODIPY is observed, that is, the phosphorescence of BODIPY. The complex shows strong absorption in the visible range (ε = 53,800  M(-1)  cm(-1) at 574 nm), which is rare for Pt(II)-acetylide complexes. The complex is dual emissive with (3)MLCT emission at 660 nm and the (3)IL emission at 770 nm. The T(1) excited state of the complex is mainly localized on the BODIPY moiety (i.e. (3)IL state, as determined by steady-state and time-resolved spectroscopy, 77 K emission spectra, and spin-density analysis). The strong visible-light-harvesting ability and long-lived T(1) excite state of the complex were used for triplet-triplet annihilation based upconversion and an upconversion quantum yield of 5.2 % was observed. The overall upconversion capability (η = ε×Φ(UC)) of this complex is remarkable considering its strong absorption. The model complex, without the BODIPY moiety, gives no upconversion under the same experimental conditions. Our work paves the way for access to transition-metal complexes that show strong absorption of visible light and long-lived (3)IL excited states, which are important for applications in photovoltaics, photocatalysis, and upconversions, etc.

Light-harvesting fullerene dyads as organic triplet photosensitizers for triplet-triplet annihilation upconversions.

Visible light-harvesting C(60)-bodipy dyads were devised as universal organic triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion. The antennas in the dyad were used to harvest the excitation energy, and then the singlet excited state of C(60) will be populated via the intramolecular energy transfer from the antenna to C(60) unit. In turn with the intrinsic intersystem crossing (ISC) of the C(60), the triplet excited state of the C(60) will be produced. Thus, without any heavy atoms, the triplet excited states of organic dyads are populated upon photoexcitation. Different from C(60), the dyads show strong absorption of visible light at 515 nm (C-1, ε = 70400 M(-1) cm(-1)) or 590 nm (C-2, ε = 82500 M(-1) cm(-1)). Efficient intramolecular energy transfer from the bodipy moieties to C(60) unit and localization of the triplet excited state on C(60) were confirmed by steady-state and time-resolved spectroscopy as well as DFT calculations. The dyads were used as triplet photosensitizers for TTA upconversion, and an upconversion quantum yield up to 7.0% was observed. We propose that C(60)-organic chromophore dyads can be used as a general molecular structural motif for organic triplet photosensitizers, which can be used for photocatalysis, photodynamic therapy, and TTA upconversions.

Visible-light harvesting iridium complexes as singlet oxygen sensitizers for photooxidation of 1,5-dihydroxynaphthalene

Visible-light harvesting cyclometalated Ir(III) complexes were used as (1)O(2) sensitizers for the photooxidation of 1,5-dihydroxynaphthalene (DHN) and substantially improved photooxidation capability was observed compared to the conventional Ir(III) complex sensitizers that show no visible light-harvesting capabilities.

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.

Red-light excitable fluorescent platinum(II) bis(aryleneethynylene) bis(trialkylphosphine) complexes showing long-lived triplet excited states as triplet photosensitizers for triplet–triplet annihilation upconversion

Symmetric and asymmetric platinum(II) bis(phosphine) bis(aryleneethynylene) complexes that show strong absorption of visible light and long-lived triplet excited states with boron dipyrromethane (Bodipy) chromophore visible light-harvesting antennae attached to the Pt(II) centres were prepared for the first time. The bisnuclear complexes Pt-2 and Pt-3, with two Pt(II) coordination centres connected to the π-core of the Bodipy ligands, show red-shifted absorption (e.g.Pt-2, λabs = 643 nm, e = 42 300 M−1 cm−1) compared to the mononuclear Pt(II) complexes (Pt-1 and Pt-4), in which only one Pt(II) coordination centre is connected to the Bodipy chromophore (e.g.Pt-1, λabs = 570 nm, e = 38 300 M−1 cm−1). The complexes are excitable with red-light, which is rare for transition metal complexes. All the complexes show fluorescence at room temperature (627–671 nm, ΦF = 1.4–6.7%), and weak phosphorescence. Long-lived Bodipy ligand-localized triplet excited states are observed for all the complexes (τT = 57.9–72.4 μs) with nanosecond transient absorption spectra, which is supported by spin density analysis. The platinum(II) bis(phosphine) bis(aryleneethynylene) complexes are used as triplet photosensitizers for the first time for red-light excited triplet–triplet annihilation (TTA) based upconversion and upconversion quantum yields of up to 19.0% are observed, and the anti-Stokes shift is up to 0.82 eV. The effects of different triplet energy transfer driving forces on the TTA upconversion with perylene and perylenebisimide as triplet acceptors are investigated. Our results are useful for the preparation of visible light-harvesting linear platinum(II) phosphine alkynyl complexes and for their applications in photocatalysis, non-linear optics and TTA upconversions.

Preparation of ketocoumarins as heavy atom-free triplet photosensitizers for triplet–triplet annihilation upconversion

A series of ketocoumarin compounds were prepared as heavy atom-free triplet photosensitizers. The photophysical properties of the compounds were studied with steady state and time-resolved spectroscopy. The compounds show absorption in the visible spectral region (molar absorption coefficients are up to ε = 136,000 M(-1) cm(-1) at 448 nm) and long-lived triplet excited states (τT = 199.7 μs) upon visible light photoexcitation. The compounds were used as triplet photosensitizers for singlet oxygen ((1)O2)-mediated photooxidation of 1,5-dihydroxylnaphthalene (DHN) to produce juglone. (1)O2 quantum yields of these compounds were determined in the range of 0.28-0.48. The ketocoumarins were also used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion, and upconversion quantum yields up to 11.3% were observed. The results are useful for preparation of heavy atom-free triplet photosensitizers and for their application in photocatalysis and TTA upconversion.