Wanhua Wu

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.

Organic Triplet Sensitizer Library Derived from a Single Chromophore (BODIPY) with Long-Lived Triplet Excited State for Triplet–Triplet Annihilation Based Upconversion

Triplet-triplet annihilation (TTA) based upconversions are attractive as a result of their readily tunable excitation/emission wavelength, low excitation power density, and high upconversion quantum yield. For TTA upconversion, triplet sensitizers and acceptors are combined to harvest the irradiation energy and to acquire emission at higher energy through triplet-triplet energy transfer (TTET) and TTA processes. Currently the triplet sensitizers are limited to the phosphorescent transition metal complexes, for which the tuning of UV-vis absorption and T(1) excited state energy level is difficult. Herein for the first time we proposed a library of organic triplet sensitizers based on a single chromophore of boron-dipyrromethene (BODIPY). The organic sensitizers show intense UV-vis absorptions at 510-629 nm (ε up to 180,000 M(-1) cm(-1)). Long-lived triplet excited state (τ(T) up to 66.3 μs) is populated upon excitation of the sensitizers, proved by nanosecond time-resolved transient difference absorption spectra and DFT calculations. With perylene or 1-chloro-9,10-bis(phenylethynyl)anthracene (1CBPEA) as the triplet acceptors, significant upconversion (Φ(UC) up to 6.1%) was observed for solution samples and polymer films, and the anti-Stokes shift was up to 0.56 eV. Our results pave the way for the design of organic triplet sensitizers and their applications in photovoltaics and upconversions, etc.

A highly selective OFF-ON red-emitting phosphorescent thiol probe with large stokes shift and long luminescent lifetime.

An OFF-ON red-emitting phosphorescent thiol probe is designed by using the (3)MLCT photophysics of Ru(II) complexes, i.e., with Ru(II) as the electron donor. The probe is non-luminescent because the MLCT is corrupted by electron transfer from Ru(II) to an intramolecular electron sink (2,4-dinitrobenzenesulfonyl). Thiols cleave the electron sink, and the MLCT is re-established. Phosphorescence at 598 nm was enhanced by 90-fold, with a 143 nm (5256 cm(-1)) Stokes shift and a 1.1 mus luminescent lifetime.

Tuning the luminescence lifetimes of ruthenium(II) polypyridine complexes and its application in luminescent oxygen sensing

Ru(Phen)(bpy)2 (1) and its new derivatives (2–5) with pyrenyl or ethynylated pyrene and phenyl units appended to the 3-position of the phenanthroline (Phen) ligand were prepared and these complexes generate long-lived room temperature phosphorescence in the red and near IR range (600–800 nm). The photophysical properties of these complexes were investigated by UV-Vis absorption, luminescence emission, transient absorption spectra and DFT/TDDFT calculations. We found the luminescence lifetime (τ)can be drastically extended by ligand modification (increased up to 140-fold), e.g. τ = 58.4 μs for complex 3 (with pyrenyl ethynylene appendents) was found, compared to τ = 0.4 μs for the reference complex 1. Ethynylated phenyl appendents alter the τ also (complex 2, τ = 2.4 μs). With pyrenyl appendents (4 and 5), lifetimes of 2.5 μs and 9.2 μs were observed. We proposed three different mechanisms for the lifetime extension of 2, 3, 4 and 5. For 2, the stabilization of the 3MLCT state by π-conjugation is responsible for the extension of the lifetime. For 3, the emissive state was assigned as an intra-ligand (IL) long-lived 3π–π* state (3IL/3LLCT, intraligand or ligand-to-ligand charge transfer), whereas a C–C single bond linker results in a triplet state equilibrium between 3MLCT state and the pyrene localized 3π–π* triplet state (3IL, e.g.4 and 5). DFT/TDDFT calculations support the assignment of the emissive states. The effects of the lifetime extension on the oxygen sensing properties of these complexes were studied in both solution and polymer films. With tuning the emissive states, and thus extension of the luminescence lifetimes, the luminescent O2 sensing sensitivity of the complexes can be improved by ca. 77-fold in solution (I0/I100 = 1438 for complex 3, vs. I0/I100 = 18.5 for complex 1). In IMPES-C polymer films, the apparent quenching constant KSVapp is improved by 150-fold from 0.0023 Torr−1 (complex 1) to 0.35 Torr−1 (complex 3). The KSVapp value of complex 3 is even higher than that of PtOEP under similar conditions (0.15 Torr−1).

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.

Ruthenium(II) polyimine-coumarin dyad with non-emissive 3IL excited state as sensitizer for triplet-triplet annihilation based upconversion.

Upconversion (UC) has attracted much attention due to its potential applications for photovoltaics, photocatalysis, nonlinear photonics, and so forth. In principle, two techniques are available for upconversion from a chemist’s perspective. The first one is to use two-photon absorption (TPA) fluorescent dyes. However, this approach suffers from fundamental drawbacks, for example, coherent light with high power density (typically MWcm , that is, 10 W cm ) is required for excitation, which is well beyond the energy level of normal light sources (the terrestrial solar radiation is ca. 0.10 W cm ). Furthermore, it is difficult to modify the molecular structures of TPA dyes to achieve a specific upconversion wavelength and at the same time to keep a high TPA cross section. A new approach for upconversion is based on triplet– triplet annihilation (TTA), which is promising for practical applications, such as photovoltaics (e.g. dye-sensitized solar cells). In this approach, a triplet sensitizer, normally a transition-metal complex with triplet excited states that are accessible upon photoexcitation (e.g. platinum(II) or palladium(II) porphyrin complexes), is used to harvest the excitation energy and transfer it to the triplet acceptor (annihilator/emitter, such as anthracene, perylene, etc.) via triplet–triplet energy transfer (TTET; see the Jablonski diagram in the Supporting Information). The excitation and emission wavelengths of TTA upconversion can be readily changed by independent selection of the triplet sensitizers and triplet acceptors, and the excitation power can be as low as a few mWcm 2 (lower than solar light). 2,5] Recently we showed that long-lived IL (intraligand) excited states are more efficient to sensitize TTA upconversion, than the normal short-lived MLCT excited states (MLCT= metal-to-ligand charge transfer). 18] However, we believe that the current understanding of TTA UC is still premature. For example, currently all the triplet sensitizers used for TTA UC are phosphorescent materials. 2,5, 12–16] However, we propose that it is unnecessary for a triplet sensitizers to be phosphorescent to sensitize a photophysical process such as TTA UC. On the contrary, the phosphorescence is actually detrimental to the TTET process as well as to upconversion because the radiative decay of the triplet excited state of the sensitizer (i.e., phosphorescence) is a decay channel which is competitive to TTET. Therefore, we envision that non-phosphorescent transition metal complexes with triplet excited states populated upon photoexcitation can sensitize the TTA UC. This new concept will greatly increase the availability of the triplet sensitizers. Herein we reported the first example of TTA upconversion with a ruthenium(II) polyimine–coumarin dyad that shows a non-emissive IL exited state and gives very weak phosphorescence but significant upconversion capability. We designed a dyad of a coumarin-containing Ru polyimine complex as the triplet sensitizer (Ru-3, Scheme 1). Coumarin was selected for its intense absorption in the visible region. To isolate the coumarin chromophore from the Ru coordination center (otherwise the system is no longer supramolecular, and the photophysics of two subunits will collapse into one), we used biphenyl and dppz (dipyr-

Styryl Bodipy-C60 Dyads as Efficient Heavy-Atom-Free Organic Triplet Photosensitizers

C60-styryl Bodipy dyads that show strong absorption of visible light (ε = 64,600 M(-1) cm(-1) at 657 nm) and a long-lived triplet excited state (τT = 123.2 μs) are prepared. The dyads were used as heavy-atom-free organic triplet photosensitizers for photooxidation of 1,5-dihydroxynaphthalene via the photosensitizing of singlet oxygen ((1)O2). The photooxidation efficiency of the dyads compared to the conventional Ir(III) complex (1)O2 photosensitizer increased 19-fold.

Accessing the Long-Lived Triplet Excited States in Bodipy-Conjugated 2-(2-Hydroxyphenyl) Benzothiazole/Benzoxazoles and Applications as Organic Triplet Photosensitizers for Photooxidations

Bodipy derivatives containing excited state intramolecular proton transfer (ESIPT) chromophores 2-(2-hydroxyphenyl) benzothiazole and benzoxazole (HBT and HBO) subunits were prepared (7-10). The compounds show red-shifted UV-vis absorption (530-580 nm; ε up to 50000 M(-1) cm(-1)) and emission compared to both HBT/HBO and Bodipy. The new chromophores show small Stokes shift (45 nm) and high fluorescence quantum yields (Φ(F) up to 36%), which are in stark contrast to HBT and HBO (Stokes shift up to 180 nm and Φ(F) as low as 0.6%). On the basis of steady state and time-resolved absorption spectroscopy, as well as DFT/TDDFT calculations, we propose that 7-9 do not undergo ESIPT upon photoexcitation. Interestingly, nanosecond time-resolved transient absorption spectroscopy demonstrated that Bodipy-localized triplet excited states were populated for 7-10 upon photoexcitation; the lifetimes of the triplet excited states (τ(T)) are up to 195 μs. DFT calculations confirm the transient absorptions are due to the triplet state. Different from the previous report, we demonstrated that population of the triplet excited states is not the result of ESIPT. The compounds were used as organic triplet photosensitizers for photooxidation of 1,5-dihydroxylnaphthalene. One of the compounds is more efficient than the conventional [Ir(ppy)(2)(phen)][PF(6)] triplet photosensitizer. Our result will be useful for design of new Bodipy derivatives, ESIPT compounds, and organic triplet photosensitizers, as well as for applications of these compounds in photovoltaics, photocatalysis and luminescent materials, etc.