Wenting Wu

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 red-emitting FRET fluorescent molecular probe derived from BODIPY for the detection of cysteine and homocysteine: An experimental and theoretical study

A red-emitting BODIPY-based fluorescent-resonance-energy-transfer (FRET) molecular probe 1 for selective detection of cysteine and homocysteine was designed. The fluorescence OFF–ON switch is triggered by cleavage of the 2,4-dinitrobenzensulfonyl (DNBS) unit from the fluorophore by thiols. The FRET energy donor (λabs = 498 nm, λem = 511 nm) is a parent BODIPY moiety and the energy acceptor is based on 4-hydroxylstyryl BODIPY moiety (λabs = 568 nm, λem = 586 nm). The unique C–C linker between the energy donor and acceptor was established using a Suzuki cross-coupling reaction. A polyether chain was also introduced into the probe to improve solubility in aqueous solution. While probe 1 itself is non-fluorescent, in the presence of cysteine or homocysteine a red emission at 590 nm is switched on (excitation at 505 nm), producing a pseudo-Stokes shift of up to 77 nm, which is in stark contrast to the small Stokes shift (ca. 10 nm) observed for typical BODIPY dyes. Excitation of the energy donor leads to the red emission from the acceptor of the probe, and demonstrates a high energy transfer efficiency. The probe was used for in vivo fluorescent imaging of cellular thiols. The fluorescence sensing mechanism of the probe and the photophysical properties of the fluorescent intermediates were fully rationalized by DFT calculations. The lack of fluorescence of probe 1 is attributed to the dark excited state S1 (oscillator strength f = 0.0007 for S0 → S1, based on the optimized S1 state geometry), which is due to the electron sink effect of the DNBS moiety. Cleavage of the DNBS moiety from the fluorophore by thiols re-establishes the emissive S1 state of the fluorophore (f = 1.4317 for S0 → S1), thus the red emission can be observed in the presence of thiols (fluorescence is turned on). The FRET effect of the probe was rationalized by DFT calculations which indicated that upon excitation into the S4 excited state (localized on the energy donor unit), the S1 state (localized on the energy acceptor, i.e. styryl-BODIPY) is populated via internal conversion (IC), thus red emission from the styryl-BODIPY energy acceptor is observed (Kashas rule).

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).

Colorimetric and ratiometric fluorescent chemosensor based on diketopyrrolopyrrole for selective detection of thiols: an experimental and theoretical study.

A colorimetric and ratiometric fluorescent thiol probe was devised with diketopyrrolopyrrole (DPP) fluorophore. The probe gives absorption and emission at 523 and 666 nm, respectively. In the presence of thiols, such as cysteine, the absorption and emission band shifted to 479 and 540 nm, respectively. Correspondingly, the color of the probe solution changed from purple to yellow, and the fluorescence changed from red to yellow. The emission intensity at 540 nm was enhanced by 140-fold. The Stokes shift of probe 1 (107 nm) is much larger than the unsubstituted DPP fluorophore (56 nm). Mass spectral analysis demonstrated that besides the expected Michael addition of thiols to the C═C bonds, the CN groups of the malonitrile moieties also react with thiols to form 4,5-dihydrothiazole structure. Probe 1 was used for fluorescence imaging of intracellular thiols. In the presence of thiols, both the green and red channel of the microscopy are active. With removal of the intracellular thiols, signal can only be detected through the red channel; thus, ratiometric bioimaging of intracellular thiols was achieved. The ratiometric response of probe 1 was rationalized by DFT calculations. Our complementary experimental and theoretical studies will be useful for design of ratiometric/colorimetric molecular probes.

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.

Coumarin phosphorescence observed with N^N Pt(II) bisacetylide complex and its applications for luminescent oxygen sensing and triplet-triplet-annihilation based upconversion.

A dbbpy platinum(II) bis(coumarin acetylide) complex (Pt-1, dbbpy = 4,4-di-tert-butyl-2,2-bipyridine) was prepared. Pt-1 shows intense absorption in the visible region (λ(abs) = 412 nm, ε = 3.23 × 10(4) M(-1) cm(-1)) compared to the model complex dbbpy Pt(II) bis(phenylacetylide) (Pt-2, λ(abs) = 424 nm, ε = 8.8 × 10(3) M(-1) cm(-1)). Room temperature phosphorescence was observed for Pt-1 ((3)IL, τ(P) = 2.52 μs, λ(em) = 624 nm, Φ(P) = 2.6%) and the emissive triplet excited state was assigned as mainly intraligand triplet excited state ((3)IL), proved by 77 K steady state emission, nanosecond time-resolved transient absorption spectroscopy and DFT calculations. Complex Pt-1 was used for phosphorescent oxygen sensing and the sensitivity (Stern-Volmer quenching constant K(SV) = 0.012 Torr(-1)) is 12-fold of the model complex Pt-2 (K(SV) = 0.001 Torr(-1)). Pt-1 was also used as triplet sensitizer for triplet-triplet-annihilation based upconversion, upconversion quantum yield Φ(UC) up to 14.1% was observed, vs. 8.9% for the model complex Pt-2.

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-

Cu–N Dopants Boost Electron Transfer and Photooxidation Reactions of Carbon Dots

The broadband light-absorption ability of carbon dots (CDs) has inspired their application in photocatalysis, however this has been impeded by poor electron transfer inside the CDs. Herein, we report the preparation of Cu-N-doped CDs (Cu-CDs) and investigate both the doping-promoted electron transfer and the performance of the CDs in photooxidation reactions. The Cu-N doping was achieved through a one-step pyrolytic synthesis of CDs with Na2 [Cu(EDTA)] as precursor. As confirmed by ESR, FTIR, and X-ray photoelectron spectroscopies, the Cuu2005species chelates with the carbon matrix through Cu-N complexes. As a result of the Cu-N doping, the electron-accepting and -donating abilities were enhanced 2.5 and 1.5u2005times, and the electric conductivity was also increased to 171.8u2005μsu2009cm(-1) . As a result of these enhanced properties, the photocatalytic efficiency of CDs in the photooxidation reaction of 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate is improved 3.5-fold after CD doping.

Long-lived room temperature deep-red/near-IR emissive intraligand triplet excited state (3IL) of naphthalimide in cyclometalated platinum(II) complexes and its application in upconversion.

[C(^)NPt(acac)] (C(^)N = cyclometalating ligand; acac = acetylacetonato) complexes in which the naphthalimide (NI) moiety is directly cyclometalated (NI as the C donor of the C-Pt bond) were synthesized. With 4-pyrazolylnaphthalimide, isomers with five-membered (Pt-2) and six-membered (Pt-3) chelate rings were obtained. With 4-pyridinylnaphthalimide, only the complex with a five-membered chelate ring (Pt-4) was isolated. A model complex with 1-phenylpyrazole as the C(^)N ligand was prepared (Pt-1). Strong absorption of visible light (ε = 21,900 M(-1) cm(-1) at 443 nm for Pt-3) and room temperature (RT) phosphorescence at 630 nm (Pt-2 and Pt-3) or 674 nm (Pt-4) were observed. Long-lived phosphorescences were observed for Pt-2 (τ(P) = 12.8 μs) and Pt-3 (τ(P) = 61.9 μs). Pt-1 is nonphosphorescent at RT in solution because of the acac-localized T(1) excited state [based on density functional theory (DFT) calculations and spin density analysis], but a structured emission band centered at 415 nm was observed at 77 K. Time-resolved transient absorption spectra and spin density analysis indicated a NI-localized intraligand triplet excited state ((3)IL) for complexes Pt-2, Pt-3, and Pt-4. DFT calculations on the transient absorption spectra (T(1) → T(n) transitions, n > 1) also support the (3)IL assignment of the T(1) excited states of Pt-2, Pt-3, and Pt-4. The complexes were used as triplet sensitizers for triplet-triplet-annihilation (TTA) based upconversion, and the results show that Pt-3 is an efficient sensitizer with an upconversion quantum yield of up to 14.1%, despite its low phosphorescence quantum yield of 5.2%. Thus, we propose that the sensitizer molecules at the triplet excited state that are otherwise nonphosphorescent were involved in the TTA upconversion process, indicating that weakly phosphorescent or nonphosphorescent transition-metal complexes can be used as triplet sensitizers for TTA upconversion.