Tao Su

University of California, Berkeley, Berkeley, CA, United States

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Publications (13)52.28 Total impact

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    ABSTRACT: 2-Acetoxymethyl-2-(3-benzoylphenyl)propionic acid (KP-OAc) was used as a model to elucidate the solvent-mediated photochemistry mechanism of Ketoprofen (KP). In solutions with a low concentration of water, KP-OAc exhibits a benzophenone-like photochemistry, reacting with water molecules through some reaction to form a ketyl radical intermediate. In neutral solutions with a high concentration of water or acidic solutions, KP-OAc undergoes a photodecarboxylation reaction with the assistance of water molecules or with the catalysis of perchloric acid to directly generate a biradical intermediate that cannot induce the phototrigger reaction to release the AcO(-) group. Therefore, the lifetime of the biradical intermediate of KP-OAc is almost same as that of the biradical intermediate formed from KP in the same kinds of solutions. However, the photodecarboxylation of KP-OAc in phosphate buffer solution directly produces the benzylic carbanion intermediate, which can induce the phototrigger reaction to release the AcO(-) group. Therefore, the lifetime of the biradical intermediate of KP-OAc is significantly shorter than the lifetime of the biradical intermediate of KP in phosphate buffer solution. Interestingly, the investigation of the photochemistry of KP-OAc not only verifies the solvent-mediated photochemistry mechanism of KP but also provides some new insight into the potential of using this kind of platform for phototrigger applications. The biradical intermediate is not the key species leading to the phototrigger reaction but the benzylic carbanion species is the key reactive intermediate that can mediate the phototrigger reaction of KP-OAc. Therefore, a change in the pH of the solutions can be utilized to switch on and switch off the photorelease reactions of KP derivative phototrigger compounds.
    Chemistry 07/2013; · 5.93 Impact Factor
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    ABSTRACT: Flurbiprofen (Fp), a nonsteroidal anti-inflammatory drug (NSAID) currently in use for arthritis pain relief and in clinical trials for metastatic prostate cancer, can induce photosensitization and phototoxicity upon exposure to sunlight. The mechanisms responsible for Fp phototoxicity are poorly understood and deserve investigation. In this study, the photodecarboxylation reaction of Fp, which has been assumed to underpin its photo-induced side effects, was explored by femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA) and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic techniques in pure acetonitrile (MeCN) solvent. Density functional theory (DFT) calculations were also performed to facilitate the assignments of transient species. The resonance Raman and DFT calculation results reveal that the neutral form of Fp was the predominant species present in MeCN. Analysis of the ultraviolet/visible absorption spectrum and results from TD-DFT calculations indicate that the second excited singlet (S2) can be excited by 266 nm light. Due to its intrinsic instability, S2 rapidly underwent internal conversion (IC) to decay to the lowest lying excited singlet (S1), which was observed in the fs-TA spectra at very early delay times. Intriguingly, three distinct pathways for S1 decay seem to co-exist. Specifically, other than fluorescence emission back to the ground state and transformation to the lowest triplet state T1 through intersystem crossing (ISC), the homolysis of the carbon α bond decarboxylation reaction proceeded simultaneously to give rise to two radical species, one being carboxyl and another being the residual, denoted as FpR. The coexistence of the triplet Fp (T1) and FpR species was verified by means of TR3 spectra along with ns-TA spectra. As a consequence of its apparent high reactivity, the FpR intermediate was observed to undergo oxidation under oxygen-saturated conditions to yield another radical species, denoted as FOR, which subsequently underwent intramolecular hydrogen transfer (IHT) and dehydroxylation (DHO) to form a final product, which could react with the carboxyl from the decarboxylation reaction to generate a minor final product. TD-DFT and transient state (TS) calculations for predicting the absorption bands and activation energies of the transient species produced in the photodecarboxylation reaction have provided valuable mechanistic insights for the assignment of the intermediate species observed in the time-resolved spectroscopy experiments reported here. The results of the time-resolved spectroscopy experiments and DFT calculations were used to elucidate the reaction mechanisms and intermediates involved in the photochemistry of Fb.
    The Journal of Physical Chemistry B 06/2013; · 3.61 Impact Factor
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    ABSTRACT: The photophysical and photochemical reactions of 3-methylbenzophenone (3-MeBP) and 4-methylbenzophenone (4-MeBP) were investigated using femtosecond transient absorption (fs-TA), nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy and density functional theory (DFT) calculations. 3-MeBP and 4-MeBP were observed to behave similar to their parent compound benzophenone (BP) in acetonitrile and isopropanol solvents. However, in acidic aqueous solutions, an unusual acid-catalyzed proton exchange reaction (denoted as the meta-methyl activation) of 3-MeBP (with a maximum efficiency at pH = 0) is detected to compete with a photohydration reaction. In contrast, only the photohydration reaction was observed for 4-MeBP under the acidic pH conditions investigated. How the meta-methyl activation takes place after photolysis of 3-MeBP in acid aqueous solutions is briefly discussed and compared to related photochemistry of other meta substituted aromatic carbonyl compounds.
    The Journal of Organic Chemistry 04/2013; · 4.56 Impact Factor
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    ABSTRACT: Although numerous donor-π-acceptor (D-π-A) type organic dyes were investigated in order to replace the ruthenium polypyridyl complexes, there have been few reports of the D-π-2A system and the related electron transfer processes. In this work, a novel D-π-2A dye (coded as B2) was designed and synthesized for applications in dye-sensitized solar cells (DSSC). Obvious intramolecular charge transfer (ICT) between the donor and acceptor takes place under photoexcitation. Three frontier LUMOs (LUMO, LUMO+1, LUMO+2) of B2 are all located on the acceptor part, which is highly favorable for intramolecular electron transfer from the donor to acceptors and enhances the electron injection into the semiconductors. DSSC based on B2 showed a maximum monochromatic incident photon-to-current efficiency (IPCE) of 68% at 425 nm and an overall power conversion efficiency of 3.62% under simulated solar light (AM 1.5G, 100 mW cm–2) irradiation. Femtosecond and nanosecond TA, and TCSPC techniques were used to monitor the photophysical properties of B2 and the electron transfer processes taking place between B2 and the semiconducting nanoparticles. It is found that electrons in the delocalized π→π* transition could be further injected into the semiconductor, while such injection process hardly happens for electrons in the localized π→π* transition.
    The Journal of Physical Chemistry C. 01/2013; 117(5):2041–2052.
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    ABSTRACT: The solvent dependent photochemistry of fenofibric acid (FA) was studied by femtosecond transient absorption and nanosecond time-resolved resonance Raman experiments and density functional theory calculations. In acetonitrile-rich solution, a typical nπ* triplet state FA ((3)FA) is formed through a highly efficient intersystem crossing and then the (3)FA species undergoes some reaction with water to generate a ketyl radical intermediate at low water concentrations. In contrast, nπ* (3)FA changes from a reaction with water to generate a ketyl radical intermediate at lower water concentrations to a decarboxylation reaction with the assistance of water molecules to produce a biradical intermediate at higher water concentrations in water-rich solutions. The decarboxylation reaction leads to the formation of the FA carbanion in 50% phosphate buffer solution and the FA carbanion is observed on the picosecond to nanosecond time scale and the cleavage of the FA carbanion gives rise to the enolate anion at later nanosecond delay times. As regards fenofibrate (FB), it only exhibits a benzophenone-like photochemistry, which consists of some reaction with water to generate a ketyl radical intermediate, being observed in the different aqueous solutions.
    Physical Chemistry Chemical Physics 12/2012; · 3.83 Impact Factor
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    ABSTRACT: The photo-decarboxylation and overall reaction mechanism of tiaprofenic acid (TPA) was investigated by femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA) and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic experiments in a neutral phosphate buffered solution (PBS). In addition, density functional theory (DFT) calculations were presented to help interpret the experimental results. Resonance Raman and DFT calculation results revealed that the deprotonated tiaprofenic acid (TPA-) form was the primary species that is photo-excited in a near neutral PBS aqueous solution. The fs-TA experimental data indicated that the lowest lying excited singlet state S1 underwent an efficient intersystem crossing process (ISC) to quickly transform into the lowest lying excited triplet state T1 that then undergoes decarboxylation to generate a triplet biradical species (TB3). Ns-TA and ns-TR3 results observed a protonation process for TB3 to produce a neutral species (TBP3) that then decayed via ISC to produce a singlet TBP species that further reacted to make the final product (DTPA). Comparison of the present results for TPA- with similar results for the deprotonated form of ketoprofen (KP-) in the literature was done to investigate how the thiophene moiety in TPA- that replaces one phenyl ring in KP- affects the reaction mechanism and photochemistry of these non-steroidal anti-inflammatory drugs (NSAIDs).
    The Journal of Physical Chemistry B 12/2012; · 3.61 Impact Factor
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    ABSTRACT: The photophysics and photochemical reactions of 2-(1-hydroxyethyl) 9,10-anthroquinone (2-HEAQ) were studied using femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy techniques and density functional theory (DFT) calculations. In acetonitrile, 2-HEAQ underwent efficient intersystem crossing to the triplet excited state ((2-HEAQ)(3)). A typical photoreduction reaction for aromatic ketones took place via production of a ketyl radical intermediate for 2-HEAQ in isopropanol. In water-containing solutions with pH values between 2 and 10, an unusual photoredox reaction reported by Wan and co-workers was detected and characterized. Observation of the protonated species in neutral and acidic aqueous solutions by fs-TA spectra indicated the carbonyl oxygen of (2-HEAQ)(3) was protonated initially and acted as a precursor of the photoredox reaction. The preference of the photoredox reaction to occur under moderate acidic conditions compared to neutral condition observed using ns-TR(3) spectroscopy was consistent with results from DFT calculations, which suggested protonation of the carbonyl group was the rate-determining step. Under stronger acidic conditions (pH 0), although the protonated (2-HEAQ)(3) was formed, the predominant reaction was the photohydration reaction instead of the photoredox reaction. In stronger basic solutions (pH 12), (2-HEAQ)(3) decayed with no obvious photochemical reactions detected by time-resolved spectroscopic experiments. Reaction mechanisms and key reactive intermediates for the unusual photoredox reaction were elucidated from time-resolved spectroscopy and DFT results. A brief discussion is given of when photoredox reactions may likely take place in the photochemistry of aromatic carbonyl-containing compounds and possible implications for using BP and AQ scaffolds for phototrigger compounds.
    Journal of the American Chemical Society 08/2012; 134(36):14858-68. · 10.68 Impact Factor
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    ABSTRACT: The decarboxylation reaction of KP in different acetonitrile-water mixtures producing a carbanion or biradical intermediate is investigated by using femtosecond transient absorption and nanosecond time-resolved resonance Raman spectroscopies to unveil the mechanism of the photochemistry of KP. The irradiation of either the neutral or anion forms of KP leads to the excited singlet state KP species transforming into a corresponding triplet state KP species via a highly efficient intersystem crossing, and then, a triplet state mediated decarboxylation reaction occurs to generate a carbanion intermediate in the phosphate buffer solutions or a biradical species in the water-rich or acidic solutions examined here.
    The Journal of Physical Chemistry B 04/2012; 116(20):5882-7. · 3.61 Impact Factor
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    ABSTRACT: Photoremovable protecting groups (PPGs) when conjugated to biological effectors forming "caged compounds" are a powerful means to regulate the action of physiologically active messengers in vivo through 1-photon excitation (1PE) and 2-photon excitation (2PE). Understanding the photodeprotection mechanism is important for their physiological use. We compared the quantum efficiencies and product outcomes in different solvent and pH conditions for the photolysis reactions of (8-chloro-7-hydroxyquinolin-2-yl)methyl acetate (CHQ-OAc) and (8-bromo-7-hydroxyquinolin-2-yl)methyl acetate (BHQ-OAc), representatives of the quinoline class of phototriggers for biological use, and conducted nanosecond time-resolved spectroscopic studies using transient emission (ns-EM), transient absorption (ns-TA), transient resonance Raman (ns-TR(2)), and time-resolved resonance Raman (ns-TR(3)) spectroscopies. The results indicate differences in the photochemical mechanisms and product outcomes, and reveal that the triplet excited state is most likely on the pathway to the product and that dehalogenation competes with release of acetate from BHQ-OAc, but not CHQ-OAc. A high fluorescence quantum yield and a more efficient excited-state proton transfer (ESPT) in CHQ-OAc compared to BHQ-OAc explain the lower quantum efficiency of CHQ-OAc relative to BHQ-OAc.
    Chemistry 04/2012; 18(22):6854-65. · 5.93 Impact Factor
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    ABSTRACT: Photoremovable protecting groups (PPGs) when conjugated to biological effectors forming “caged compounds” are a powerful means to regulate the action of physiologically active messengers in vivo through 1-photon excitation (1PE) and 2-photon excitation (2PE). Understanding the photodeprotection mechanism is important for their physiological use. We compared the quantum efficiencies and product outcomes in different solvent and pH conditions for the photolysis reactions of (8-chloro-7-hydroxyquinolin-2-yl)methyl acetate (CHQ-OAc) and (8-bromo-7-hydroxyquinolin-2-yl)methyl acetate (BHQ-OAc), representatives of the quinoline class of phototriggers for biological use, and conducted nanosecond time-resolved spectroscopic studies using transient emission (ns-EM), transient absorption (ns-TA), transient resonance Raman (ns-TR2), and time-resolved resonance Raman (ns-TR3) spectroscopies. The results indicate differences in the photochemical mechanisms and product outcomes, and reveal that the triplet excited state is most likely on the pathway to the product and that dehalogenation competes with release of acetate from BHQ-OAc, but not CHQ-OAc. A high fluorescence quantum yield and a more efficient excited-state proton transfer (ESPT) in CHQ-OAc compared to BHQ-OAc explain the lower quantum efficiency of CHQ-OAc relative to BHQ-OAc.
    Central European Journal of Chemistry 04/2012; · 1.17 Impact Factor
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    ABSTRACT: The properties of photo-generated reactive species, holes and electrons in bulk TiO2 (anatase) film and nano-sized TiO2 were studied and their effects towards decomposing pollutant dye methyl orange (MO) were compared by transient absorption spectroscopies. The recombination of holes and electrons in nanosized TiO2 was found to be on the microseconds time scale consistent with previous reports in the 10 literature. However, in bulk TiO2 film, the holes and electrons recombination was found to be on the order of picoseconds due to ultra fast free electrons. The time-correlated single-photon counting (TCSPC) technique combined with confocal fluorescence microscopy revealed that the fluorescence intensity of MO is at first enhanced noticeably by TiO2 under UV excitation and soon afterwards weakened dramatically, with the lifetime prolonged. Photo-generated holes in nano-sized TiO2 can directly oxidize 15 MO on the time scale of nanoseconds, while free electrons photo-generated in bulk TiO2 film can directly inject into MO on the order of picoseconds. Through cyclic voltammetry measurements, it was found that MO can be reduced at -0.28 V and oxidized at 1.4 V (vs SCE ) and this provides thermodynamic evidence for MO to be degraded by electrons and holes in TiO2. Through comparison of the hole-scavenging effect of MO and water, it was found that in polluted water When MO is above 1.6×10-4 M, the degradation is mainly due to direct hole oxidation process, while below 1.6×10-4 20 M, hydroxyl oxidation competes strongly and might exceed the hole oxidation.
    Physical Chemistry Chemical Physics 01/2012; · 3.83 Impact Factor
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    ABSTRACT: Hydrogen abstraction reaction of fenofibric acid (FA) in acetonitrile and isopropyl alcohol solvents was studied by femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy experiments. The singlet excite state ((1)FA) (nπ*) with a maximum transient absorption at 352 nm observed in the fs-TA experiments undergoes efficient intersystem crossing (ISC) to convert into a nπ* triplet state FA ((3)FA) that exhibits two transient absorption bands at 345 and 542 nm. The nπ* (3)FA species does not decay obviously within 3000 ps. In the ns-TR(3) experiments, the nπ* (3)FA is also observed and completely decays by 120 ns. Compared with the triplet states of benzophenone (BP) and ketoprofen (KP), the nπ* (3)FA species seems to have a much higher hydrogen abstraction reactivity so that (3)FA decays fast and generates a FA ketyl radical like species. In isopropyl alcohol solvent, the nπ* (3)FA exhibits similar reactivity and promptly abstracts a hydrogen from the strong hydrogen donor isopropyl alcohol solvent to generate a ketyl radical intermediate. With the decay of the FA ketyl radical, no light absorption transient (LAT) intermediate is observed in isopropyl alcohol solvent although such a LAT species was observed after similar experiments for BP and KP. Comparison of the ns-TR(3) spectra for the species of interest with results from density functional theory calculations were used to elucidate the identity, structure, properties, and major spectral features of the intermediates observed in the ns-TR(3) spectra. This comparison provides insight into the structure and hydrogen abstraction reactivity of the triplet states of BP derivatives.
    The Journal of Physical Chemistry A 11/2011; 115(49):14168-74. · 2.77 Impact Factor
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    ABSTRACT: Hydrogen abstraction reaction of fenofibric acid (FA) in acetonitrile and isopropanol solvents was studied by femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy experiments. The singlet excite state (1FA) (nπ*) state with a maximum transient absorption at 352 nm observed in the fs-TA experiments undergoes efficient intersystem crossing (ISC) to convert into a nπ* triplet state FA (3FA) that exhibits two transient absorption bands at 345 and 542 nm. The nπ* 3FA species does not decay obviously within 3000 ps. In the ns-TR3 experiments, the nπ* 3FA is also observed and completely decays by 120 ns. Compared with the triplet states of benzophenone (BP) and ketoprofen (KP), the nπ* 3FA species seems to have a much higher hydrogen abstraction reactivity so that the 3FA decays fast and generates a FA ketyl radical like species. In isopropanol solvent, the nπ* 3FA exhibits similar reactivity and promptly abstracts a hydrogen from the strong hydrogen donor isopropanol solvent to generate a ketyl radical intermediate. With the decay of the FA ketyl radical, no light absorption transient (LAT) intermediate is observed in isopropanol solvent although such a LAT species was observed after similar experiments for BP and KP. Comparison of the ns-TR3 spectra for the species of interest with results from density functional theory calculations were used to elucidate the identity, structure, properties and major spectral features of the intermediates observed in the ns-TR3 spectra. This comparison provides insight into the structure and hydrogen abstraction reactivity of the triplet states of BP derivatives.
    The Journal of Physical Chemistry A 01/2011; · 2.77 Impact Factor