Shaomin Ji

Triplet–triplet annihilation based upconversion: from triplet sensitizers and triplet acceptors to upconversion quantum yields

Triplet–triplet annihilation (TTA) is a promising upconversion approach due to its low excitation power density (solar light is sufficient), high upconversion quantum yield, readily tunable excitation/emission wavelength and strong absorption of excitation light. This review focuses on the reported TTA based upconversion examples, the challenges that are facing the developments of TTA upconversion and the design rationales for the triplet sensitizers and triplet acceptors.

Tuning the Intramolecular Charge Transfer of Alkynylpyrenes: Effect on Photophysical Properties and Its Application in Design of OFF—ON Fluorescent Thiol Probes

Green and yellow-emitting 1,6- and 1,8-bis(phenylethynyl) pyrenes (dyes 7, 8, 9, and 10) with different intramolecular charge transfer (ICT) feature were synthesized and the effect of ICT on the photophysical properties of these derivatives were studied by UV-vis absorption spectra, fluorescence emission spectra, and DFT/TDDFT calculations. For the dyes with electron-pushing group (e.g., -dimethylamino, dye 8 and dye 10), structureless and solvent polarity-sensitive fluorescence emission spectra were observed. Conversely, dye with electron-withdrawing group (e.g., -CN, dye 7) shows structured and solvent polarity-independent emission spectra. OFF-ON fluorescent thiol probes 11 and 12 with 2,4-dinitrobenzenesulfonyl protected ethynylpyrene fluorophore were designed based on DFT/TDDFT calculations, which predicts dark state (S(1)) for these thiol probes (e.g., oscillator strength f = 0.0086 for S(1)<--S(0) transition of the probe 11). This dark state is induced by the ICT effect with ethynylated pyrene fluorophore as electron donor and 2,4-dinitrobenzenesulfonyl unit as electron acceptor. Cleavage of the 2,4-dinitrobenzenesulfonyl unit by thiol releases the free fluorophore, for which the lowest-lying excited state S(1) is no longer a dark state, but an emissive state (f = 0.9776 for S(1)<--S(0) transition). These theoretical predictions on the photophysical properties of the molecular probes were fully proved by experimental results. Our results demonstrated that the fluorescence OFF-ON switching of this kind of thiol probe is due to the termination of the ICT effect (which quenches the emission, by a dark S(1) state) by cleavage of the 2,4-dinitrobenzenesulfonyl unit (as acceptor of ICT effect) with thiols, not the re-establishment of the D-pi-A feature of the fluorophore. These investigation on the pyrene derived green-emitting fluorophores and the DFT/TDDFT calculation aided probe design suggest that future application of these results may prove useful toward the rational design of fluorophores or fluorescent probes with predetermined photophysical properties.

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).

Rational Design of d-PeT Phenylethynylated-Carbazole Monoboronic Acid Fluorescent Sensors for the Selective Detection of α-Hydroxyl Carboxylic Acids and Monosaccharides

We have synthesized three new phenylethynylated carbazole boronic acid sensors, which were predicted to display novel d-PeT fluorescence transduction (PeT, photoinduced electron transfer; fluorophore as the electron donor of the electron transfer, ET) by DFT/TDDFT calculations. The d-PeT effect is characterized by a lower background fluorescence at acidic pH than at neutral pH, which is in stark contrast to the normal a-PeT effect (fluorophore as the electron acceptor of the ET) that shows a strong and undesired background fluorescence at acidic pH. Our experimental results confirmed the theoretical predictions and d-PeT was observed for two of the sensors (with p-dimethylaminophenylethynyl substitution at 6- position of the carbazole core). For the third sensor (with phenylethynyl substitution at 6- position of the carbazole core), however, not d-PeT but rather the normal a-PeT was observed. The discrepancy between the DFT/TDDFT calculations and the experimental observations can be rationalized using free energy changes (Rehm-Weller equations) and the rate constants for the ET (k(ET), Marcus equation). These new d-PeT boronic acid sensors show improved photophysical properties compared to the known d-PeT sensor reported previously by us. In particular, the fluorescence transduction efficiency of the new sensors was improved 8-fold when compared to the known d-PeT boronic acid sensors. Novel fluorescence enhancement/reduction was observed for one of the sensors upon binding with mandelic acid or tartaric acid at pH 5.6. The effect of pH as well as the bonding with analytes on the emission of the sensors were rationalized using DFT/TDDFT calculations. We believe that rational sensor design aided by DFT/TDDFT calculations as well as using free energy changes and electron transfer rate constants to study the emission properties of PeT sensors will become an essential tool in the design of new fluorophores or fluorescent sensors with predetermined photophysical properties.

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).

Styryl-BODIPY based red-emitting fluorescent OFF-ON molecular probe for specific detection of cysteine.

We have synthesized a styryl boron-dipyrromethene (BODIPY)/2,4-dinitrobenzenesulfonyl (DNBS) dyad based red-emitting molecular probe for specific detection of cysteine among the biological thiols. The probe shows intensive absorption at 556 nm and the probe is non-fluorescent. The DNBS moiety can be cleaved off by thiols, the red emission of the BODIPY fluorophore at 590 nm is switched on, with an emission enhancement of 46-fold. The probe shows good specificity toward cysteine over other biological molecules, such as glutathione and amino acids. The emission of the probe is pH-independent in the physiological pH range. The probe is used for fluorescent imaging of cellular thiols. Theoretical calculations based on density functional theory (DFT) were used to elucidate the fluorescence sensing mechanism of the probe, which indicate a dark excited state (S(1)) for the probe but an emissive excited state (S(1)) for the cleaved product (i.e. the fluorophore).

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.